Basic Structure Of IP Addresses

IP (Internet Protocol) addresses are used to identify hosts on the campus Internet, a Cornell network that ties into the Internet, a global network. If the computer is attached to Cornell's network, it needs an IP address to be recognized as part of the campus Internet.

IP addresses are constructed according to a set of specific rules so that hosts on any part of the Internet can communicate with each other. This document describes IP addresses only as they apply to Cornell's campus network. (If you want to know more about Internet addressing, refer to Internetworking with TCP/IP: Principles, Protocols, and Architecture by Douglas Comer, Prentice Hall).

An IP address consists of a 32-bit binary number, which is typically presented as four decimal numbers (one for each 8-bit byte) separated by decimal points. For example,

Internet addresses at Cornell have three parts:

* network address
* subnet address
* host address

When you configure a host for sub-subnetting, you are primarily concerned about the host address, but some understanding of the network address and subnet address is useful.

* Network Address
Cornell has four addresses for its backbone networks. They are,,, and The latter is used only by the Cornell University College. These addresses are assigned to Cornell. Cornell cannot change the first two parts of each address, but is free to use the last two parts in any way it chooses in order to identify Local Area Networks (subnets) and hosts that are connected to the campus Internet.

* Subnet Address
The subnet address is the address given to your Local Area Network (LAN). Cornell's system provides for 254 LANs connected to each of the main networks. So, for example, if your LAN is identified on the network as, a possible subnet addresses (or LAN address) might be The third number, 21, identifies the subnet.

* Host Address
The host address is the address given to the workstation, other computer, or device that is connected to the LAN. Cornell's system provides for 256 host addresses on each LAN. So, for example, if your host is identified on the LAN as a possible host address is The last number, 58, identifies the host.

Not all 256 numbers are available as host addresses on any given LAN. Zero (0) and 255 are reserved for broadcast purposes. (Hosts are set up to "grab" any message marked with their own address or a broadcast address; for example, if your host address is and it "sees" a message addressed to, it will grab the message. In this way, hosts can send messages to large groups without having to know each address on their LAN.)

One (1) is reserved for the gateway/router that sits between the LAN and next network level. The numbers 2-5 are reserved by CIT for diagnostic and management use.

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