What exactly is an intranet? It’s one of those terms that’s more thrown around than understood and has become more of a buzzword than a commonly understood idea. Simply put, an intranet is a private network with Internet technology used as the underlying architecture. An intranet is built using the Internet’s TCP/IP protocols for communications. TCP/IP protocols can be run on many hardware platforms and cabling schemes. The underlying hardware is not what makes an intranet-it’s the software protocols that matter.
Intranets can co-exist with other local area networking technology. In many companies, existing “legacy systems” including mainframes, Novell networks, minicomputers, and various databases, are being integrated into an intranet. A wide variety of tools allows this to happen. Common Gateway Interface (CGI) scripting is often used to access legacy databases from an intranet. The Java programming language can be used to access legacy databases as well.
With the enormous growth of the Internet, an increasing number of people in corporations use the Internet to communicate with the outside world, gather information, and do business. It didn’t take long for people to recognize that the components that worked so well on the Internet could be equally valuable internally, and that is why intranets are becoming so popular. Some corporations do not have TCP/IP networks, the protocol required to access the resources of the Internet. Creating an intranet in which all the information and resources can be used seamlessly has many benefits. TCP/IP-based networks make it easy for people to remotely access the network, such as from home or traveling. Dialing into an intranet in this way is much like connecting to the Internet, except that you’re connecting to a private network instead of a public Internet provider. Interoperability between networks is another substantial bonus.
Security systems separate an intranet from the Internet. A company’s intranet is protected by firewalls-hardware and software combinations that allow only certain people to access the intranet for specific purposes.
Intranets can be used for anything that existing networks are used for and more. The ease of publishing information on the World Wide Web has made them popular places for posting corporate information such as company news or company procedures. Corporate databases with easy-to-build front-ends use the Web and programming languages such as Java.
Intranets allow people to work together more easily and more effectively. Software known as groupware is another important part of intranets. It allows people to collaborate on projects; share information; do video conferencing, and establish security procedures for production work. Free server and client software and many services, like newsgroups, stimulated the Internet’s growth. The consequence of that growth stimulated and fueled the growth of intranets. The ease with which information can be shared and with which people can communicate with one another will continue to drive the building of intranets.
A Global View of an Intranet
An intranet is a private corporate or educational network that uses the Internet’s TCP/IP protocols for its underlying transport. The protocols can run on various network hardware and co-exist with other network protocols, such as IPX. People from inside an intranet can get at the larger Internet resources. Still, those on the Internet cannot get into the intranet, allowing only restricted access from the Internet.
- Video conferencing is an important application that requires sending massive quantities of data. Intranets can be built using components that allow the extremely high bandwidths required for transferring such information.
- Often an intranet is composed of several different networks inside a corporation that all communicate with one another via TCP/IP. These separate networks are often referred to as subnets.
- Software that allows people to communicate via e-mail and public message boards and collaborate on work using workgroup software is among the most powerful intranet programs. Applications that allow different corporate departments to post information and fill out corporate forms, such as timesheets, and tap into corporate financial information are very popular.
- Much of the software used on intranets are standard, off-the-shelf Internet software such as the Netscape Navigator and the Microsoft Explorer Web browsers. And customized programs are often built using the Java programming language and CGI scripting.
Read More Articles :
- Intranets can also be used to allow companies to do business-to-business transactions, such as ordering parts, sending invoices, and making payments. These intranet-to-intranet transactions need never go out over the public Internet for extra security but can travel over private leased lines instead.
- Intranets are a powerful system for allowing a company to do business online, such as allowing anyone on the Internet to order products. When someone orders a product on the Internet, information is sent securely from the public Internet to the company’s intranet, where the order is processed and completed.
- To protect sensitive corporate information and to ensure that hackers don’t damage computer systems and data, security barriers are called firewalls to protect an intranet from the Internet. Firewall technology uses a combination of routers, servers, and other hardware and software to allow people on an intranet to use Internet resources. Still, it blocks outsiders from getting into the intranet.
- Many intranets have to connect to “legacy systems”-hardware and databases built before an intranet was constructed. Legacy systems often use older technology not based on the intranet’s TPC/IP protocols. There are a variety of ways in which intranets can tie to legacy systems. A common way is to use CGI scripts to access the database information and pour that data into HTML formatted text, making it available to a Web browser.
- Information sent across an intranet is sent to the proper destination by routers, which examine each TCP/IP packet for the IP address and determine the packet’s destination. It then sends the packet to the next router closest to the destination. If the packet is to be delivered to an address on the same subnetwork of the intranet it was sent from; the packet may be delivered directly without going through any other routers. If it is sent to another subnetwork on the intranet, it will be sent to another internal router address. If the packet is to be sent to a destination outside the intranet, in other words, to an Internet destination, the packet is sent to a router that connects to the Internet.
How TCP/IP and IPX Work on Intranets
What distinguishes an intranet from any other kind of private network is that it is based on TCP/IP-the same protocols that apply to the Internet. TCP/IP refers to two protocols that work together to deliver data: the Transmission Control Protocol (TCP) and the Internet Protocol (IP). When you send information across an intranet, the data is broken into small packets. The packets are sent independently through a series of switches called routers. Once all the packets arrive at their destination, they are recombined into their original form. The Transmission Control Protocol breaks the data into packets and recombines them on the receiving end. The Internet Protocol handles the data routing and makes sure it gets sent to the proper destination.
- In some companies, there may be a mix of TCP/IP-based intranets and networks based on other networking technology, such as NetWare. In that instance, an intranet’s TCP/IP technology can be used to send data between NetWare or other networks, using a technique called IP tunneling. We’ll look at data being sent from one NetWare network to another via an intranet in this instance. NetWare networks use the IPX (Internet Packet Exchange) protocol to deliver data, and TCP/IP networks can’t recognize that protocol. To get around this, when an IPX packet is to be sent across an intranet, it is first encapsulated inside an IP packet by a NetWare server specifically for and dedicated to providing the IP transport mechanism for IPX packets.
- Data sent within an intranet must be broken up into packets of less than 1,500 characters each. TCP breaks the data into packets. As it creates each packet, it calculates and adds a checksum to the packet. The checksum is based on the byte values, that is, the precise amount of data in the packet.
- Each packet, along with the checksum, is put into separate IP wrappers or “envelopes.” These wrappers contain information that details exactly where the data is to be sent on the intranet or the Internet. All of the wrappers for a given piece of data have the same addressing information so that they can all be sent to the same location for reassembly.
- The packets travel between networks by intranet routers. Routers examine all IP wrappers and look at their addresses. These routers determine the most efficient path for sending each packet to its final destination. Since the traffic load on an intranet often changes, the packets may be sent along different routes, and the packets may arrive out of order. If the router sees the address inside the intranet, the packet may be sent directly to its destination, or it may instead be sent to another router. If the address is located out on the Internet, it will be sent to another router to be sent across the Internet.
- As the packets arrive at their destination, TCP calculates a checksum for each packet. It then compares this checksum with the checksum that has been sent in the packet. If the checksums don’t match, TCP knows that the data in the packet has been corrupted during transmission. It then discards the packet and asks that the original packet is retransmitted.
- TCP includes the ability to check packets and to determine that all the packets have been received. When all the non-corrupt packets are received, TCP assembles them into their original, unified form. The header information of the packets relays the sequence of how to reassemble the packets.
- An intranet treats the IP packet like any other and routes the packet to the receiving NetWare network. On the receiving NetWare network, a NetWare TCP/IP server decapsulates the IP packet. It discards the IP packet and reads the original IPX packet. It can now use the IPX protocol to deliver the data to the proper destination.
How the OSI Model Works
A group called the International Standards Organization (ISO) put together the Open Systems Interconnect (OSI) Reference Model, a model that describes seven layers of protocols for computer communications. These players don’t know or care what is in adjacent layers. Each layer, essentially, only sees the reciprocal layer on the other side. The sending application layer sees and talks to the application layer on the destination side. That conversation takes place irrespective of, for example, what structure exists at the physical layer, such as Ethernet or Token Ring. TCP combines the OSI model’s application, presentation, and session layers into one, which is also called the application layer.
- The application layer refers to application interfaces, not programs like word processing. MHS (Message Handling Service) is such an interface, and it operates at this level of the OSI model. Again, this segmentation and interface approach means that various email programs can be used on an intranet so long as they conform to the MHS standard at this application interface level.
- The presentation layer typically simply provides a standard interface between the application layer and the network layers. This type of segmentation allows for the great flexibility of the OSI model since applications can vary endlessly. Still, as long as the results conform to this standard interface, the applications need not be concerned with any other layers.
- The session layer allows for the communication between sender and destination. These conversations avoid confusion by speaking in turn. A token is passed to control and to indicate which side is allowed to speak. This layer executes transactions, like saving a file. If something prevents it from completing the save, the session layer, which has a record of the original state, returns to the original state rather than allowing a corrupt or incomplete transaction to occur.
- The transport layer segments the data into acceptable packet sizes and is responsible for the data integrity of packet segments. Several levels of service can be implemented at this layer, including segmenting and reassembly, error recovery, flow control, and others.
- The IP wrapper is put around the packet at the network or Internet layer. The header includes the source and destination addresses, the sequence order, and other data necessary for correct routing and rebuilding at the destination.
- The data-link layer frames the packets, for example, for use with the PPP (Point to Point). It also includes the logical link portion of the MAC sublayer of the IEEE 802.2, 802.3, and other standards.
- Ethernet and Token Ring are the two most common physical layer protocols. They function at the MAC (Media Access Control) level and move the data over the cables based on the physical address on each NIC (Network Interface Card). The physical layer includes the physical components of the IEEE 802.3 and other specifications.
How TCP/IP Packets Are Processed
Protocols such as TCP/IP determine how computers communicate over networks such as the Internet. These protocols work in concert with each other and are layered on top of one another in what is commonly referred to as a protocol stack. Each layer of the protocol is designed to accomplish a specific purpose on sending and receiving computers. The TCP stack combines the application, presentation, and session layers into a single layer called the application layer. Other than that change, it follows the OSI model. The illustration below shows the wrapping process that occurs to transmit data.
- The TCP application layer formats the data being sent so that the layer below it, the transport layer, can send the data. The TCP application layer performs the equivalent actions that the top three OSI layers perform: the application, presentation, and session layers.
- The next layer down is the transport layer, which is responsible for transferring the data, and ensures that the data sent and the data received are, in fact, the same data. In other words, that there have been no errors introduced during the sending of the data. TCP divides the data it gets from the application layer into segments. It attaches a header to each segment. The header contains information that will be used on the receiving end to ensure that the data hasn’t been altered en route and that the segments can be properly recombined into their original form.
- The third layer prepares the data for delivery by putting them into IP datagrams and determining the proper Internet address for those datagrams. The IP protocol works in the Internet layer, also called the network layer. It puts an IP wrapper with a header onto each segment. The IP header includes information such as the IP address of the sending and receiving computers, the length of the datagram, and the sequence order of the datagram. The sequence order is added because the datagram could conceivably exceed the size allowed for network packets and need to be broken into smaller packets. Including the sequence, the order will allow them to be recombined properly.
- The Internet layer checks the IP header and checks to see whether the packet is a fragment. If it is, it puts together fragments back into the original datagram. It strips off the IP header and then sends the datagram to the transport layer.
- The transport layer looks at the remaining header to decide which application layer protocol-TCP or UDP-should get the data. Then the proper protocol strips off the header and sends the data to the receiving application.
- The application layer gets the data and performs, in this case, an HTTP request.
- The next layer down, the data link layer, uses protocols such as the Point-to-Point Protocol (PPP) to put the IP datagram into a frame. This is done by putting a header-the third header, after the TCP header and the IP header and a footer around the IP datagram to frame it. The frame header includes a CRC check that checks for errors in the data as the data travels over the network.
- The data-link layer ensures that the CRC for the frame is right and that the data hasn’t been altered while it was sent. It strips off the frame header and the CRC and sends the frame to the Internet layer.
- The packet travels through the stack on the receiving computer but in the opposite order from which the packet was created. In other words, it starts at the bottom layer and moves its way up through the protocol stack. As it moves up, each layer strips off the header information added to the TCP/IP stack of the sending computer.
- The final layer is the physical network layer, which specifies the physical characteristics of the network being used to send data. It describes the actual hardware standards, such as the Ethernet specification. The layer receives the frames from the data link layer and translates the IP addresses into the hardware addresses required for the specific network being used. Finally, the layer sends the frame over the network.
- The physical network layer receives the packet. It translates the hardware address of the sender and receiver into IP addresses. Then it sends the frame up to the data link layer.
How Bridges Work
Bridges are hardware and software combinations that connect different parts of a single network, such as different sections of an intranet. They connect local area networks (LANs). They are generally not used, however, for connecting entire networks, for example, for connecting an intranet to the Internet, or an intranet to an intranet, or to connect an entire subnetwork to an entire subnetwork. To do that, more sophisticated pieces of technology called routers are used.
- When there is a great amount of traffic on an Ethernet local area network, packets can collide with one another, reduce the network’s efficiency, and slow down network traffic. Packets can collide because so much of the traffic is routed among all the workstations on the network.
- To cut down on the collision rate, a single LAN can be subdivided into two or more LANs. For example, a single LAN can be subdivided into several departmental LANs. Most of the traffic in each departmental LAN stays within the department LAN, and so it needn’t travel through all the workstations on all the LANs on the network. In this way, collisions are reduced. Bridges are used to link the LANs. The only traffic that needs to travel across bridges is traffic bound for another LAN. Any traffic within the LAN needs not to travel across a bridge.
- Each packet of data on an intranet has more information in it than just the IP information. It also includes addressing information required for other underlying network architecture, such as for an Ethernet network. Bridges look at this outer network addressing information and deliver the packet to the proper address on a LAN.
- Bridges consult a learning table that has the addresses of all the network nodes in it. If a bridge finds that a packet belongs on its own LAN, it keeps the packet inside the LAN. If it finds that the workstation is on another LAN, it forwards the packet. The bridge constantly updates the learning table as it monitors and routes traffic.
- Bridges can connect LANs in a variety of different ways. They can connect LANs using serial connections over traditional phone lines and modems, over ISDN lines, and direct cable connections. CSU/DSU units are used to connect bridges to telephone lines for remote connectivity.
- Bridges and routers are sometimes combined into a single product called a router. A router handles both bridging and routine tasks. If the data needs to be sent only to another LAN on the network or subnetwork, it will act only as a bridge delivering the data based on the Ethernet address. If the destination is another network entirely, it will act as a router, examine the IP packets, and route the data based on the IP address.
How Intranet Routers Work
Just as routers direct traffic on the Internet, sending information to its proper destination, and routers on an intranet perform the same function. Routers-equipment that combines hardware and software can send the data to a computer on the same sub-network inside the intranet, to another network on the intranet, or outside to the Internet. They do this by examining header information in IP packets and then sending the data on its way. Typically, a router will send the packet to the next router closest to the final destination, which in turn sends it to an even closer router, and so on, until the data reaches its intended recipient.
- A router has input ports for receiving IP packets and output ports for sending those packets toward their destination. When a packet comes to the input port, the router examines the packet header and checks the destination in it against a routing table, a database that tells the router how to send packets to various destinations.
- Based on the information in the routing table, the packet is sent to a particular output port, which sends the packet to the next closest router to the packet’s destination.
- If packets come to the input port more quickly than the router can process them, they are sent to a holding area called an input queue. The router then processes packets from the queue in the order they were received. If the number of packets received exceeds the queue’s capacity (called the length of the queue), packets may be lost. When this happens, the TCP protocol on the sending and receiving computers will have the packets re-sent.
- In a simple intranet that is a single, completely self-contained network, and in which there are no connections to any other network or the intranet, only minimal routine need be done. So the routing table in the router is exceedingly simple with very few entries and is constructed automatically by a program called config.
- In a slightly more complicated intranet composed of several TCP/IP-based networks and connects to a limited number of TCP/IP-based networks, static routing will be required. In static routing, the routing table has specific ways of routing data to other networks. Only those pathways can be used. Intranet administrators can add routes to the routing table. Static routing is more flexible than minimal routing, but it can’t change routes as network traffic changes and isn’t suitable for many intranets.
- In more complex intranets, dynamic routing will be required. Dynamic routing is used to permit multiple routes for a packet to reach its final destination. Dynamic routing also allows routers to change the way they route information based on network traffic on some paths and routers. In dynamic routing, the routing table is called a dynamic routing table and changes as network conditions change. The tables are built dynamically by routing protocols and so constantly change according to network traffic and conditions.
- There are two broad types of routing protocols: interior and exterior. Interior routing protocols are typically used on internal routers inside an intranet that routes traffic bound only inside the intranet. A common interior routing protocol is the Routing Information Protocol (RIP). Exterior protocols are typically used for external routers on the Internet. The aêcommon exterior protocol is the Exterior Gateway Protocol (EGP).
Intranets come in different sizes. In a small company, an intranet can be composed of only a handful of computers. A medium-sized business may include dozens or hundreds of computers. And in a large corporation, there may be thousands of computers spread across the globe, all connected to a single intranet. When intranets get large, they need to be subdivided into individual subnets or subnetworks.
To understand how subnetting works, you first need to understand IP addresses. Every IP address is a 32-bit numeric address that uniquely identifies a network and a specific host on that network. The IP address is divided into two sections: the network section, called the need, and the host section, called the hosting.
Each 32-bit IP address is handled differently, according to what class of network the address refers to. There are three main classes of network addresses: Class A, Class B, and Class C. In some classes, more of the 32-bit address space is devoted to the need, while in others, more of the address space is devoted to the hosted. In a Class A network, the net is composed of 8 bits, while the host is composed of 24 bits. In a Class B network, both the nett and the host are composed of 16 bits. In a Class C network, the need comprises 24 bits, while the host is composed of 8 bits. There’s a simple way of knowing what class a network is in. If the first number of the IP address is less than 128, the network is a Class A address. If the first number is from 128 to 191, it’s a Class B network. If the first number is from 192 to 223, it’s a Class C network. Numbers above 223 are reserved for other purposes. The smaller the need, the fewer networks that can be subnetted, but the larger number of hosts on the network. A Class A rating is best for large networks, while a Class C is best for small ones.
To create a subnet, the demarcation line on the IP address is moved between the need and the host to give the need more bits to work with and take away bits from the host. To do this, a special number called a subnet mask is used.
Subnetting is used when intranets grow over a certain size, and they begin to have problems. One problem is the management of host IP addresses ensuring that every computer on the network has a proper, up-to-date host address and that old host addresses are put out of use until needed in the future. In a corporation spread out over several locations or across the world, it’s difficult, if not impossible, to have one person responsible for managing the host addresses at every location and department in the company.
Another problem has to do with a variety of hardware limitations of networks. Dissimilar networks may all be part of an intranet. An intranet may have some Ethernet sections, other sections that are Token Ring networks, and conceivably other sections that use different networking technologies altogether. There is no easy way for an intranet router to link these dissimilar networks together and route the information to the proper places.
Another set of problems has to do with the physical limitations of network technology. There are some strict limitations on how far cables can extend in the network in some kinds of networks. In other words, you can’t go over a certain distance of cabling without using repeaters or routers. A “thick” Ethernet cable, for example, can only be extended to 500 meters, while a “thin” Ethernet cable can only go to 300 meters. Routers can link these cables together so that an intranet can be extended well beyond those distances. But when that is done, each length of wire is essentially considered its own subnetwork.
Yet, one more set of problems has to do with the volume of traffic that travels across an intranet. Often in a corporation, in a given department, most of the traffic is intradepartmental traffic. In other words, mail and other data that people within a department send to each other. The volume of traffic outside to other departments is considerably less. What’s called for is to confine intradepartmental traffic inside the departments and cut down on the amount of data that needs to be routed and managed across the entire intranet.
Subnetting solves all these problems and more. When an intranet is divided into subnets, one central administrator doesn’t have to manage every aspect of the entire intranet. Instead, each subnet can take care of its own administration. That means smaller organizations within the larger organization can take care of problems such as address management and various troubleshooting chores. If divisions or departments subnet an intranet, it means that each division or department can guide the development of its own network while adhering to general intranet architecture. Doing this allows departments or divisions more freedom to use technology to pursue their business goals.
Subnets also get around problems that arise when an intranet has different kinds of network architecture, such as Ethernet and Token Ring technologies. Normally, there is no subnetting-a router that can’t link these different networks together because they don’t have their own addresses. However, if each of the different networks is its own subnet and has its own network address-routers, it can then link them together and properly route intranet traffic.
Subnetting can also cut down on the traffic traveling across the intranet and its routers. Since much network traffic may be confined within departments, having each department be its own subnet means that all that traffic need never cross an intranet router and cross the intranet-it will stay within its own subnet.
Subnetting can also increase the security on an intranet. If the payroll department, for example, were on its own subnet, then much of its traffic would not have to travel across an intranet. Having its data traveling across the intranet could mean that someone could conceivably hack into the data to read it. Confining the data to its own subnet makes that much less likely to happen.
Dividing an intranet into subnets can also make the entire intranet more stable. If an intranet is divided in this way, if one subnet goes down or is often unstable, it won’t affect the rest of the intranet.
This all may sound rather confusing. To see how it’s done, let’s look at a network and see how to use the IP address to create subnets. Let’s say we have a Class B network. That network is assigned the address of 188.8.131.52. When a network is given an address, it is assigned the netid numbers-in this case, the 130.97-and it can assign the host numbers (in this case, 0.0) in any way it chooses.
The 184.108.40.206 network is a single intranet. It’s getting too large to manage, though, and we’ve decided to divide it into two subnets. What we do is fairly straightforward. We take a number from the host field and use it to identify each of the subnets. So one subnet gets the address 220.127.116.11, and the other gets the address 18.104.22.168. Individual machines on the first subnet get addresses of 22.214.171.124, 126.96.36.199, and so on. Individual machines on the second subnet get addresses of 188.8.131.52, 184.108.40.206, and so on.
Sounds simple. But we have a problem. The Internet doesn’t recognize 220.127.116.11 and 18.104.22.168 as separate networks. It treats them both as 22.214.171.124 since the “1” and “2” that we’re using as a netid is only known to the Internet as a hosted. So our intranet router will not be able to route incoming traffic to the proper network.
To solve the problem, a subnet mask is used. A subnet mask is a 32-bit number in IP form used by intranet routers and hosts to help routers understand how to route information to the proper subnet. There is still only one network on the outside Internet, but the subnet mask allows routers inside the intranet to send traffic to the proper host.
A subnet mask is a number such as 255.255.255.0 (the built-in default for Class C addresses; the Class B default is 255.255.0.0, and the default for Class A is 255.0.0.0). A router takes the subnet mask and applies that number to the IP number of incoming mail to the network by using it to perform a calculation. The resulting IP number will route mail to the proper subnet and then to a particular computer on the subnet. For consistency, everyone in a particular intranet will use the same subnet mask.
Subnetting an Intranet
When intranets are over a certain size or are spread over several geographical locations, it becomes difficult to manage them as a single network. To solve the problem, the single intranet can be subdivided into several subnets, subsections of an intranet that make them easier to manage. To the outside world, the intranet still looks as if it’s a single network.
- If you’re building an intranet and want it to be connected to the Internet, you’ll need a unique IP address for your intranet network, which the InterNIC Registration Services will handle. You can have three classes of the intranet: Class A, Class B, or Class C. Generally, a Class A rating is best for the largest networks, while a Class C is best for the smallest. A Class A network can be composed of 127 networks and 16,777,214 nodes on the network. A Class B network can be composed of 16,383 networks and a total of 65,534 nodes. A Class C network can be composed of 2,097,151 networks and 254 nodes.
- When an intranet is assigned an address, it is assigned the first two IP numbers of the Internet numeric address (called the need field), and the remaining two numbers (called the host field) are left blank so that the intranet itself can assign them, such as 126.96.36.199. The hosted field consists of several subnets and a host number.
- When an intranet is connected to the Internet, a router handles sending packets into the intranet from the Internet. In our example, all incoming mail and data come to a router for a network with the need of 188.8.131.52.
- When intranets grow, for example, if there is a department located in another building, city, or country, there needs to be some way to manage network traffic. It may be impractical and physically impossible to route all the data necessary among many different computers spread across a building or the world. A second network called a subnetwork or subnet needs to be created.
- To have a router handle all incoming traffic for a subnetted intranet, the first byte of the host field is used. The bits that are used to distinguish among subnets are called subnet numbers. In our example, there are two subnets on the intranet. To the outside world, there appears to be only one network.
- Each computer on each subnet gets its own IP address, as in a normal intranet. The combination of the need field, the subnet number, and then finally a host number forms the IP address.
- The router must be informed that the host field in subnets must be treated differently than non-subnetted hosting fields. Otherwise, it won’t be able to properly route data. To do this, a subnet mask is used. A subnet mask is a 32-bit number such as 255.255.0.0 used in concert with the numbers in the hosting field. When a calculation is performed using the subnet mask and the IP address, the router knows where to route the mail. The subnet mask is put in people’s network configuration files.