IP address

A 32-bit logical address for a host on a TCP/IP network. Each host on a TCP/IP network needs a unique IP address for communication to take place reliably on the network.

How It Works

IP addresses are usually expressed in four-octet, dotted-decimal form—w.x.y.z—in which each octet ranges in value from 0 to 255 (with some restrictions). The IP address of a host is partitioned by the network’s subnet mask into two parts, a network ID and a host ID.

IP addresses belong to certain classes according to their first octet, as defined in the following table. The actual distinguishing feature of each class is the pattern of high-order bits in the first octet, but it is easier to remember these classes by their first octet decimal numbers.

IP Address Classes

IP Address Class Possible First Octet Used For
Class A 1–126 Very large networks
Class B 128–191 Medium to large networks
Class C 192–223 Small networks
Class D 224–239 Multicasting
Class E 240–255 Reserved (experimental)

IP addresses whose first octet is 127 represent the loopback address and are used for troubleshooting purposes only, not for naming hosts.

NOTE

Networks that are directly connected to the Internet must have their IP addresses assigned by the Internet Network Information Center (InterNIC) or some other authority. Businesses usually obtain these addresses through their local Internet service provider (ISP). However, firewall and proxy server combinations, which are popular on today’s networks, hide a network’s IP addresses from other hosts on the Internet. These private networks can use any IP addresses they choose, although InterNIC recommends the following IP address blocks for private networks:

Note the following considerations for valid IP addressing:

subnet mask

A 32-bit number that is used to partition IP addresses into a network ID and a host ID. Subnet masks are used by TCP/IP services and applications to determine whether a given IP address on an internetwork is a local network address or a remote network address.

How It Works

Subnet masks are represented as four-octet dotted-decimal numbers, just as IP addresses are, except that the most common values for an octet in a subnet mask are 0 and 255. In binary notation, decimal 0 represents the octet 00000000, and decimal 255 represents 11111111. A subnet mask consists of 32 binary digits, the first n of which are 1s and the remaining of which are 0s. When the subnet mask is logically ANDed with a 32-bit IP address of a TCP/IP host, the result is the network ID of the host—the portion of the host’s IP address that identifies which network the host is on. When the inverse of the subnet mask (for example, NOT mask) is logically ANDed with the IP address of the host, the result is the host ID of the host—the portion of the host’s IP address that uniquely identifies the host on its network.

.207 = 1 1 0 0 1 1 1 1

...61 = 0 0 1 1 1 1 0 1

...16 = 0 0 0 0 1 0 0 0

.119 = 0 1 1 1 0 1 1 1

For example, consider the IP address 207.61.16.119 and the subnet mask 255.255.255.0. Converting these two numbers to binary and ANDing them gives the host’s Network ID:

Host = 11001111 00111101 00010000 01110111	= 207.61.16.119
Mask = 11111111 11111111 11111111 00000000	= 255.255.255.0
AND  = 11001111 00111101 00010000 00000000	= 207.61.16.0
     = 207.61.16.0 = network ID

Taking the logical NOT of the subnet mask and ANDing it with the host’s IP address gives the host’s Host ID:

Host     = 11001111 00111101 00010000 01110111	= 207.61.16.119
NOT Mask = 00000000 00000000 00000000 11111111	=   0. 0. 0. 1
     AND = 00000000 00000000 00000000 01110111	=   0. 0. 0.119
         = 0.0.0.119 = host ID

Two types of subnet masks are used in TCP/IP networking:

NOTE

The default subnet masks for IP address classes A, B, and C are shown in the following table. The table also shows how these subnet masks would partition an IP address such as w.x.y.z into a network ID and a host ID portion.

Default Subnet Masks for IP Addresses

Class Default Subnet Mask Network ID Host ID
A 255.0.0.0 w x.y.z
B 255.255.0.0 w.z y.z
C 255.255.255.0 w.x.y z

Some TCP/IP configuration programs, such as those for Ascend routers, use a different notation for specifying subnet masks. They append a suffix of the form /n to the host’s IP address to indicate the subnet mask, in which n equals the number of binary 1s in the subnet mask. Thus, for example, 207.61.16.119/24 signifies 207.61.16.119/255.255.255.0.

subnetting

The process of partitioning a single TCP/IP network into a number of separate networks called subnets. These subnets are then joined using routers. Advantages of subnetting a network include the following:

How It Works

To subnet a TCP/IP network, you take the assigned network ID and borrow bits from the host ID to establish a group of subnet IDs, one for each subnet. The more bits you borrow, the more subnets you produce, but the fewer the number of possible hosts for each subnet. The borrowing process also defines a unique custom subnet mask for the network.

For example, consider a class B network that uses the network ID 172.16.0.0. If this network needs to be subnetted into six subnets, you can accomplish this using a custom subnet mask of 255.255.224.0. Each subnet can be shown to support a maximum of 8190 hosts. The IP address blocks for the six subnets are as follows:

TIP

Manual calculation of custom subnet masks and subnet IDs is tedious. You can download numerous subnetting calculators from the Internet, some of them for free. To use these calculators to subnet your network, you must first determine how many subnets you need and the maximum number of hosts on each subnet.

ipconfig

A TCP/IP utility that displays the current TCP/IP configuration settings for each network interface card (NIC) on a Microsoft Windows NT or Windows 2000 system. The ipconfig command is often one of the first commands you use to check the status of the connection when you experience communication problems on a TCP/IP network.

How It Works

When you type ipconfig at a command prompt, the following information is displayed for each NIC:

You can display additional information, including the host name, physical address, and Dynamic Host Configuration Protocol (DHCP), Windows Internet Name Service (WINS), and Domain Name System (DNS) configuration, using ipconfig /all.

You can release and renew IP addresses obtained by DHCP using ipconfig /release and ipconfig /renew.

IP telephony

An umbrella term for technologies that allow the transmission of voice and video over TCP/IP internetworks instead of the traditional Public Switched Telephone Network (PSTN). Many vendors have produced platforms and software for IP telephony, but the technology is still in its growth stage and standards are not yet clearly established.

The advantage of IP telephony is that it allows voice communication to be closely integrated with data transmission over corporate networks and allows long-distance communication to be established over the Internet instead of through private long-distance carriers such as Sprint and MCI WorldCom. The primary difficulty with IP telephony is that the existing Internet Protocol (IP) internetwork is connectionless and suffers from latency that can cause annoying glitches (pauses) in voice and video transmission over IP packets. This happens because TCP/IP was not originally designed as a connection-oriented service capable of specific levels of quality of service (QoS), whereas the PSTN, with its circuit-switched connections, provides just that.

IPv4

The current 32-bit IP addressing scheme used on TCP/IP networks worldwide. Because the number of hosts connecting to the Internet has skyrocketed in recent years, unique IP addresses are gradually running out. A new scheme called IPv6 has been proposed and is currently under review by the Internet community. However, with most corporate networks now hiding their networks behind firewalls, the pressure to move to IPv6 has lessened because companies can choose any network ID they want for their private network. The only assigned IP addresses they require from their Internet service provider (ISP) are for the public interfaces on their firewall machines. At this point, IPv4 seems to be firmly entrenched in the networking world for at least the next few years.

Internet Protocol (IP)

A TCP/IP network layer protocol for addressing and routing packets of data between hosts on a TCP/IP network. Internet Protocol (IP) is a connectionless protocol that provides best-effort delivery using packet-switching services.

How It Works

IP does not guarantee delivery of data. The responsibility for guaranteeing delivery and sending acknowledgments lies with the higher transport-level protocol Transmission Control Protocol (TCP).

The structure of an IP packet is shown in the following diagram. Some of the more important header fields include

IP packets are routed in the following fashion:

bus topology

A networking topology that connects networking components along a single cable or that uses a series of cable segments that are connected linearly. A network that uses a bus topology is referred to as a “bus network.” Bus networks were the original form of Ethernet networks, using the 10Base5 cabling standard. Bus topology is used for

Bus topology is the cheapest way of connecting computers to form a workgroup or departmental LAN, but it has the disadvantage that a single loose connection or cable break can bring down the entire LAN.

NOTE

A hub or concentrator on an Ethernet network is really a collapsed bus topology. Physically, the network appears to be wired in a star topology, but internally the hub contains a collapsed bus, creating a configuration called a star-wired bus. However, in this case, a failure in one of the cables does not affect the remaining network.

TIP

Communication problems on bus networks might indicate that the bus ends are improperly terminated. A break in the cable will produce a similar result, since the ends of the break are not terminated. Use a cable tester to determine the nature and location of the problem.

See alsomesh topology, ring topology, star topology

routing table

An internal table that a computer or router uses to determine which router interface to send packets to, based on their destination network addresses. Microsoft Windows platforms automatically build their own routing tables, which are used to determine whether to forward specific packets to

To view the internal TCP/IP routing table on a computer running Windows 2000, Windows NT, Windows 98, or Windows 95, type route print at the command prompt.

A typical routing table looks like the following:

Active Routes:
Network Address   Netmask     Gateway Address     Interface  Metric
127.0.0.0       255.0.0.0        127.0.0.1        127.0.0.1    1
172.16.8.0      255.255.255.0    172.16.8.50      172.16.8.50  1
172.16.8.50     255.255.255.255  127.0.0.1        127.0.0.1    1
172.16.255.255  255.255.255.255  172.16.8.50      172.16.8.50  1
224.0.0.0       224.0.0.0        172.16.8.50      172.16.8.50  1
255.255.255.255 255.255.255.255  172.16.8.50      172.16.8.50  1

This computer has a single network interface card (NIC) with the address 172.16.8.50. The columns of this table are as follows:

NOTE

In Microsoft Exchange Server, the routing table is the internal table that defines how messages can be routed to other sites in the Exchange organization and to foreign mail systems through installed connectors and gateways. This routing table is generally known as the Gateway Address Routing Table (GWART).