Section 4.2. Basic Information

Regardless of whether you decide to connect your network to the Internet, one thing is certain: you will build your enterprise network using the TCP/IP protocols. All TCP/IP networks, whether or not they connect to the Internet, require the same basic information to configure the physical network interface. As we will see in Chapter 6, the network interface needs an IP address and may also need a subnet mask and broadcast address. The decision of whether to connect to the Internet affects how you obtain the values needed to configure the interface. In this section, we look at how the network administrator arrives at each of the required values.

4.2.1 Obtaining an IP Address

Every interface on a TCP/IP network must have a unique IP address. If a host is part of the Internet, its IP address must be unique within the entire Internet. If a host's TCP/IP communications are limited to a local network, its IP address only needs to be unique locally. Administrators whose networks will not be connected to the Internet can select an address from RFC 1918, Address Allocation for Private Intranets, which lists network numbers that are reserved for private use.[1] The private network numbers are:

[1] The address used in this book ( is treated as an official address, but it is a private network number set aside for use by non-connected enterprise networks. Feel free to use this address on your network if it will not be connected to the Internet.

  • Network (10/8 prefix) is a 24-bit block of addresses.

  • Networks to (172.16/12 prefix) is a 20-bit block of addresses.

  • Networks to (192.168/16 prefix) is a 16-bit block of addresses.

The disadvantage of using a network address from RFC 1918 is that you may have to change your address in the future if you connect your full network to the Internet. The advantages to choosing a private network address are:

  • It's easy. You do not have to apply for an official address or get anyone's approval.

  • It's friendly. You save address space for those who need to connect to the Internet.

  • It's free. RFC 1918 addresses cost nothingofficial addresses cost money.

If you do choose an address from RFC 1918, the hosts on your network can still have access to systems on the Internet. But it will take some effort. You'll need a network address translation (NAT) box or a proxy server. NAT is available as a separate piece of hardware or as an optional piece of software in some routers and firewalls. It works by converting the source address of datagrams leaving your network from your private address to your official address. Address translation has several advantages:

  • It conserves IP addresses. Most network connections are between systems on the same enterprise network. Only a small percentage of systems need to connect to the Internet at any one time. Therefore, far fewer official IP addresses are needed than the total number of systems on an enterprise network. NAT makes it possible for you to use a large address space from RFC 1918 for configuring your enterprise network while using only a small official address space for Internet connections.

  • It reduces address spoofing, a security attack in which a remote system pretends to be a local system. The addresses in RFC 1918 cannot be routed over the Internet. Therefore, even if a datagram is routed off your network toward the remote system, the fact that the datagram contains an RFC 1918 destination address means that the routers in the Internet will discard the datagram as a martian .[2]

    [2] A martian is a datagram with an address that is known to be invalid.

  • It eliminates the need to renumber your hosts when you connect to the Internet.

Network address translation also has disadvantages:


NAT may add cost for new hardware or optional software. However, these costs tend to be very low.


Address translation adds overhead to the processing of every datagram. When the address is changed, the checksum must be recalculated. Furthermore, some upper-layer protocols carry a copy of the IP address that also must be converted.


Routers never modify the addresses in a datagram header, but NAT does. This might introduce some instability. Additionally, protocols and applications that embed addresses in their data may not function correctly with NAT.


NAT limits the use of end-to-end encryption and authentication. Authentication schemes that include the header within the calculation do not work because the router changes the addresses in the header. Encryption does not work if the encrypted data includes the source address.

Proxy servers provide many of the same advantages as NAT boxes. In fact, these terms are often used interchangeably. But there are differences. Proxy servers are application gateways originally created as part of firewall systems to improve security. Internal systems connect to the outside world through the proxy server, and external systems respond to the proxy server. Proxy servers are application-specific. A network might have one proxy web server and another proxy FTP servereach server dedicated to serving connections for one type of application. Therefore, the difference between NAT boxes and proxy servers is that NAT maps IP addresses regardless of the application; the true proxy server focuses on one application.

Proxy servers often have added security features. Address translation can be done at the IP layer. Proxy services require the server to handle data up to the application layer. Security filters can be put in proxy servers that filter data at all layers of the protocol stack.

Given the differences discussed here, network address translation servers should scale better than proxy servers, and proxy servers should provide better security. However, over time these technologies have merged and are now largely indistinguishable. Before you decide to use either NAT or proxy services, make sure they are suitable for your network needs.

Combining NAT with a private network address gives every host on your network access to the outside world, but it does not allow outside users access into your network. For that, you need to obtain an official IP address. Obtaining an official network address

Networks that are fully connected to the Internet must obtain official network addresses. An official address is needed for every system on your network that is directly accessible to remote Internet hosts. Every network that communicates with the Internet, even those that use NAT, have at least one official address, although that address may not be permanently assigned. The first step toward obtaining a block of addresses is to determine how many addresses you need.

Determining your "organizational type" helps you assess your address needs and how you should satisfy those needs. RFC 2901, Administrative Internet Infrastructure Guide, describes four different organizational types:

Internet end user

A small- to medium-sized organization focused on connecting itself to the Internet. This could be as small as a single user connecting to the Internet with a dynamic address assigned by the ISP's DHCP server, or as large as a network of thousands of hosts using NAT on the enterprise network and official addresses on a limited number of publicly accessible systems. What categorizes this organizational type is that it wants to use the Internet while limiting the number of systems it makes available to remote users. "Internet end user" organizations obtain official addresses from their ISP. From the point of view of the Internet, all Internet end user organizations appear small because they use only a limited number of official addresses.

High-volume end user

A medium-sized to large organization that distributes official addresses to systems throughout its network. This type of organization tends to have a distributed management under which divisions within the overall organization are allowed to make systems remotely accessible. "High-volume end user" organizations usually satisfy their address requirements through their ISP or a Local Internet Registry. If the organization needs more than 8,000 addresses, it may go directly to a Regional Internet Registry. While in reality a high-volume end user organization may not be any larger than an Internet end user organization, it appears to be larger from the point of view of the Internet because it exposes more systems to the Internet.

Internet Service Provider

An organization that provides Internet connection services to other organizations and provides those organizations with official addresses. Even an ISP connects to the Internet in some way. If it connects through another ISP, that ISP is its upstream provider. The upstream provider assigns addresses to the ISP. If it connects directly to a network access point (NAP), as described in Chapter 2, the ISP requests addresses from the Local Internet Registry or the Regional Internet Registry.

Local Internet Registry

An organization that provides addresses to ISPs. In effect, a Local Internet Registry is an organization that provides addresses to other organizations that provide addresses. A Local Internet Registry must obtain its addresses from a Regional Internet Registry.

RFC 2901 lists four organizational types in order to be thorough, but most organizations are either Internet end users or high-volume end users. In all likelihood, your organization is one of these, and you will obtain all of your addresses from your ISP.

Your ISP has been delegated authority over a group of network addresses and should be able to assign you a network number. If your local ISP cannot meet your needs, perhaps the ISP's upstream provider can. Ask your local ISP who it receives service from and ask that organization for an address. If all else fails, you may be forced to go directly to an Internet registry. If you are forced to take your request to a registry, you will need to take certain steps before you make the application.

You need to prepare a detailed network topology. The topology must include a diagram that shows the physical layout of your network and highlights its connections to the Internet. You should include network engineering plans that, in addition to diagramming the topology, describe:

  • Your routing plans, including the protocols you will use and any constraints that forced your routing decisions.

  • Your subnetting plans, including the mask you will use and the number of networks and hosts you will have connected during the next year. RFC 2050, Internet Registry IP Allocation Guidelines, suggests the following details in your subnet plan:

    • A table listing all subnets.

    • The mask for each subnet. The use of variable-length subnet masks (VLSMs) is strongly encouraged. VLSMs are described later in this chapter under "Defining a Subnet Mask."

    • The estimated number of hosts.

    • A descriptive remark explaining the purpose of each subnet.

The biggest challenge is accurately predicting your future requirements for addresses. If you have previously been assigned an address block, you may be required to provide a history of how that address block was used. Even if it is not requested by the Internet registry, a history can be a helpful tool for your own planning. Additionally, you will be asked to prepare a network deployment plan. This plan typically shows the number of hosts you currently have that need official addresses and the number you expect to have in six months, one year, and two years.

One factor used to determine how much address space is needed is the expected utilization rate. The expected utilization rate is the number of hosts assigned official addresses divided by the total number of hosts possible for the network. The deployment plans must show the number of hosts that will be assigned addresses over a two-year period. The total number of possible hosts can be estimated from the total number of employees in your organization and the number of systems that have been traditionally deployed per employee. Clearly you need to have a global knowledge of your organization and its needs before applying for an official address assignment.

In addition to providing documentation that justifies the address request, obtaining an official address requires a formal commitment of resources. Most address applications require at least two contacts: an administrative contact and a technical contact. The administrative contact should have the authority to deal with administrative issues ranging from policy violations to billing disputes. The technical contact must be a skilled technical person who can deal with technical problems and answer technical questions. The registries require that these contacts live in the same country as the organization that they represent. You must provide the names, addresses, telephone numbers, and email addresses of these people. Don't kid yourselfthese are not honorary positions. These people have targets on their backs when things go wrong.

The registry includes this contact information in the whois database, which provides publicly available contact information about the people responsible for networks. Once your name is in the whois database, you're given a NIC handle, which is a unique identifier linked to your whois database record. For example, my NIC handle is cwh3. Many official applications request your NIC handle.

In addition to human resources, you need to commit computer resources. You should have systems set up, running, and ready to accept the new addresses before you apply for official addresses.

When all of the background work is done, you're ready to present your case to an Internet registry. A three-level bureaucracy controls the allocation of IP addresses:


The Internet Assigned Numbers Authority allocates large blocks of addresses to regional Internet registries.

Regional Internet Registry

Regional Internet Registries (IRs) have been given authority by the IANA to allocate addresses within a large region of the world. There are three IRs:


The Asian Pacific Network Information Center has address allocation authority for Asia and the Pacific region.


The American Registry for Internet Numbers has address allocation authority for the Americas.


Reseaux IP Europeens has address allocation authority for Europe.

Local Internet Registry

Local IRs are given authority, either by IANA or by a regional IR, to allocate addresses within a specific area. An example might be a national registry or a registry created by a consortium of ISPs.

Regardless of how much address space you need, you should start at the bottom of the hierarchy and work your way up. Always start with your local ISP. If they cannot handle your needs, ask them if there is a local IR that can help you. As a last resort, take your request to the regional IR that serves your part of the world.

If you're in the APNIC region, first fill out the membership application. The APNIC membership application is available at Once you become a member of APNIC, you can request an address.

ARIN does not require that you become a member before applying for an address. If you're a high-volume end user, use the application form at to apply for an address. If you're an ISP, use In either case, send the completed application to

End user organization in the RIPE region must use a local IR. RIPE only allocates addresses to local IRs that are members of RIPE. End user organizations cannot apply to RIPE for address allocations. See the document for more information.

Regardless of where your network is located, the most important thing to remember is that most organizations never have to go through this process because they do not want to expose the bulk of their computers to the Internet. For security reasons, they use private address numbers for most systems and have only a limited number of official IP addresses. That limited number of addresses can usually be provided by a local ISP. Obtaining an IN-ADDR.ARPA domain

When you obtain an official IP address, you should also apply for an domain. This special domain is sometimes called a reverse domain. Chapter 8 contains more information about how the domain is set up and used, but basically the reverse domain maps numeric IP addresses into domain names. This is the reverse of the normal domain name lookup process, which converts domain names to addresses. If your ISP provides your name service or assigned you an address from a block of its own addresses, you may not need to apply for an domain on your own. Check with your ISP before applying. If, however, you obtain a block of addresses from a Regional Internet Registry, you probably need to get your own domain. If you do need to get a reverse domain, you will register it with the same organization from which you obtained your address assignment.

  • For address blocks obtained from APNIC, use the form and mail the completed form to

  • For address blocks obtained from ARIN, use the form and mail the completed form to

  • For address blocks obtained from RIPE, a domain object needs to be entered into the RIPE database. Mail the completed object to

As an example, assume that your network is located in the RIPE region. You would need to provide the information needed to create a RIPE domain object for your network. The domain object for the RIPE database illustrates the type of information that is required to register a reverse domain. The RIPE database object has ten fields:


This is the domain name. How reverse domain names are derived is described in detail in Chapter 8, but the name is essentially the address reversed with added to the end. For our 172.16/16 address allocation, the reverse domain name is


A text description of the domain. For example, "The address allocation for"


The NIC handle of the administrative contact.


The NIC handle of the technical contact.


The NIC handle of the domain administrator, also called the zone contact.


The name or address of the master server for this domain.


The name or address of a slave server for this domain.


For RIPE, this third server is always


The email address of the maintainer who submitted this database object and the date it was submitted.


For addresses allocated by RIPE, the value of this field is always RIPE.

Again, the most important thing to note about reverse address registration is that most organizations don't have to do this. If you obtain your address from your ISP, you probably do not have to take care of this paperwork yourself. These services are one of the reasons you pay your ISP.

4.2.2 Assigning Host Addresses

So far we have been discussing network numbers. Our imaginary company's network was assigned network number The network administrator assigns individual host addresses within the range of IP addresses available to the network address; i.e., our administrator assigns the last two bytes of the four-byte address.[3] The portion of the address assigned by the administrator cannot have all bits 0 or all bits 1; i.e., and are not valid host addresses. Beyond these two restrictions, you're free to assign host addresses in any way that seems reasonable to you.

[3] The range of addresses is called the address space.

Network administrators usually assign host addresses in one of two ways:

One address at a time

Each individual host is assigned an address, perhaps in sequential order, through the address range.

Groups of addresses

Blocks of addresses are delegated to departments within the organization, which then assign the individual host addresses.

The assignment of groups of addresses is most common when the network is subnetted and the address groups are divided along subnet boundaries. But assigning blocks of addresses does not require subnetting. It can be just an organizational device for delegating authority. Delegating authority for groups of addresses is often very convenient for large networks, while small networks tend to assign host addresses one at a time. No matter how addresses are assigned, someone must retain sufficient central control to prevent duplication and to ensure that the addresses are recorded correctly on the domain name servers.

Addresses can be assigned statically or dynamically. Static assignment is handled through manually configuring the boot file on the host computer. Dynamic address assignment is always handled by a server, such as a DHCP server. One advantage of dynamic address assignment is that the server will not accidentally assign duplicate addresses. Thus, dynamic address assignment is desirable not only because it reduces the administrator's workload but also because it reduces errors.

Before installing a server for dynamic addressing, make sure it is useful for your purposes. Dynamic PPP addressing is useful for servers that handle many remote dial-in clients that connect for a short duration. If the PPP server is used to connect various parts of the enterprise network and has long-lived connections, dynamic addressing is probably unnecessary. Likewise, the dynamic address assignment features of DHCP are of most use if you have mobile systems in your network that move between subnets and therefore need to change addresses frequently. See Chapter 6 for information on PPP, and Chapter 3 and Chapter 9 for details about DHCP.

Clearly, you must make several decisions about obtaining and assigning addresses. You also need to decide what bit mask will be used with the address. In the next section we look at the subnet mask, which changes how the address is interpreted.

4.2.3 Defining the Subnet Mask

As the prefix number indicates, a network address is assigned with a specific address mask. For example, the prefix of 16 in the network address means that ARIN assigned our imaginary network the block of addresses defined by the address and the 16-bit mask[4] Unless you have a reason to change the interpretation of your assigned network number, you do not have to define a subnet mask. Chapter 2 described the structure of IP addresses and touched upon the reasons for subnetting. The decision to subnet is commonly driven by topological or organizational considerations.

[4] Even though is an RFC 1918 private network number, this text treats as if it were an officially assigned network number, for the sake of example.

The topological reasons for subnetting include:

Overcoming distance limitations

Some network hardware has very strict distance limitations. The original 10 Mbps Ethernet is the most common example. The maximum length of a "thick" Ethernet cable is 500 meters; the maximum length of a "thin" cable is 300 meters; the total length of a 10 Mbps Ethernet, called the maximum diameter, is 2500 meters.[5] If you need to cover a greater distance, you can use IP routers to link a series of Ethernet cables. Individual cable still must not exceed the maximum allowable length, but using this approach, every cable is a separate Ethernet. Therefore the total length of the IP network can exceed the maximum length of an Ethernet.

[5] The faster the Ethernet, the smaller its network diameter. For this reason, high-speed Ethernet technologies use switches instead of a daisy chain cable to connect nodes.

Interconnecting dissimilar physical networks

IP routers can be used to link networks that have different and incompatible underlying network technologies. Figure 4-1 later in this chapter shows a central token ring subnet,, connecting two Ethernet subnets, and

Filtering traffic between networks

Local traffic stays on the local subnet. Only traffic intended for other networks is forwarded through the gateway.

Subnetting is not the only way to solve topology problems. Networks are implemented in hardware and can be altered by changing or adding hardware, but subnetting is an effective way to overcome these problems at the TCP/IP level.

Of course, there are non-technical reasons for creating subnets. Subnets often serve organizational purposes such as:

Simplifying network administration

Subnets can be used to delegate address management, troubleshooting, and other network administration responsibilities to smaller groups within the overall organization. This is an effective tool for managing a large network with a limited staff. It places the responsibility for managing the subnet on the people who benefit from its use.

Recognizing organizational structure

The structure of an organization (or simply office politics) may require independent network management for some divisions. Creating independently managed subnets for these divisions is preferable to having them go directly to an ISP to get their own independent network numbers.

Isolating traffic by organization

Certain organizations may prefer to have their local traffic isolated to a network that is primarily accessible only to members of that organization. This is particularly appropriate when security is involved. For example, the payroll department might not want its network packets on the engineering network where some clever person could figure out how to intercept them.

Isolating potential problems

If a certain segment is less reliable than the remainder of the net, you may want to make that segment a subnet. For example, if the research group puts experimental systems on the network from time to time or experiments with the network itself, this part of the network will be unstable. You would make it a subnet to prevent experimental hardware or software from interfering with the rest of the network.

The network administrator decides if subnetting is required and defines the subnet mask for the network. The subnet mask has the same form as an IP address mask. As described in Chapter 2, it defines which bits form the "network part" of the address and which bits form the "host part." Bits in the "network part" are turned on (i.e., 1) while bits in the "host part" are turned off (i.e., 0).

The subnet mask used on our imaginary network is This mask sets aside 8 bits to identify subnets, which creates 256 subnets. The network administrator has decided that this mask provides enough subnets and that the individual subnets have enough hosts to effectively use the address space of 254 hosts per subnet. The upcoming Figure 4-1 shows an example of this type of subnetting. Applying this subnet mask to the addresses and causes them to be interpreted as the addresses of two different networks, not as two different hosts on the same network.

Once a mask is defined, it must be disseminated to all hosts on the network. There are two ways this is done: manually, through the configuration of network interfaces, and automatically, through configuration protocols like DHCP. Routing protocols can distribute subnet masks, but in most environments host systems do not run routing protocols. In this case, every device on the network must use the same subnet mask because every computer believes that the entire network is subnetted in exactly the same way as its local subnet.

Because routing protocols distribute address masks for each destination, it is possible to use variable-length subnet masks (VLSMs). Using variable-length subnet masks increases the flexibility and power of subnetting. Assume you wanted to divide into three networks: one network of 110 hosts, one network of 50 hosts, and one network of 60 hosts. Using traditional subnet masks, a single subnet mask would have to be chosen and applied to the entire address space. At best, this would be a compromise. With variable-length subnet masks you could use a mask of to create subnets of 126 hosts for the large subnet, and a mask of to create subnets of 62 hosts for the smaller subnets. VLSMs, however, require that every router on the network knows how to store and use the masks and runs routing protocols that can transmit them. (See Chapter 7 for more information on routing.) Routing is an essential part of a TCP/IP network. Like other key components of your network, routing should be planned before you start configuration.