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IP Addresses and Subnets at Cornell

This page will give you a basic understanding of the structure of IP addresses and subnets at Cornell.

What Are IP Addresses and How Are They Used?

IP (Internet Protocol) addresses are used to identify hosts on the campus network that in turn ties into the Internet, a global network. A networked computer must have an IP address assigned to the computer to be recognized as part of the network.

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, 128.253.21.58.

Internet addresses at Cornell have three parts:

Network Address

Cornell has several addresses for its backbone networks. The addresses are 128.84.0.0, 128.253.0.0, 132.236.0.0, 192.35.82.0, 192.122.235.0, and 192.122.236.0. 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 128.253.0.0, a possible subnet addresses (or LAN address) might be 128.253.21.0. 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 128.253.21.0 a possible host address is 128.253.21.58. 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 128.253.21.58 and it "sees" a message addressed to 128.253.21.255, 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.

IP Addressing Space

This addressing scheme has worked well for Cornell, but it has some limitations:

  • Each of Cornell's fiber backbones can have no more than 256 LANs attached to them.
  • Each LAN can have no more than 256 (249 if reserved addresses are taken into account) hosts. Most LANs are constructed with far fewer than the maximum number of hosts addresses available.
  • Ethernet LAN performance is reduced with a large number (100 or more) of connections. Performance is most affected by how people are using the LAN: a small number of heavy users can bog down the performance of any LAN. Therefore, the limitations on the number of host addresses hasn't been and isn't expected to be a problem for most LANs at Cornell.

Classless Inter-Domain Routing (CIDR) helps us bypass limitations on the number of IP addresses available on our network.

How Does Subnetting Work?

Each subnet address at Cornell is assigned a "subnet mask." A subnet mask defines how many bits are used for the network address and how many for the host address.

The subnet mask address is 255.255.255.0, and it currently is the same for all LANs. If you convert the subnet mask address to its binary form, it looks like this:

Subnet mask: 11111111 11111111 11111111 00000000

If you convert our example host address (128.253.21.58) to its binary form, it looks like this:

Host address: 10000000 11111101 00010101 00111010

Together they look like this:

Subnet mask: 11111111 11111111 11111111 00000000

Host address: 10000000 11111101 00010101 00111010

The subnet mask when shown this way, as an overlay on the host address, essentially tells the computer which part of the IP address is a network address and which part is a host address. Everything in the host address that corresponds to a 1 in the subnet mask is a network address and everything in the host address that corresponds to a 0 in the subnet mask is a host address.

Understanding Static vs. Dynamic Addressing

If you use static addressing on your network, this means you assign each host a permanent IP address. If you use dynamic addressing, the hosts use any available address within a range you specify. The information below explains some of the advantages and disadvantages of static and dynamic addressing.

Before you begin reconfiguring your applications, you should decide which scheme you plan to use: static, dynamic, or a combination. CIT recommends that you use static addressing; it helps you, as the network administrator, keep track of machines and figure out which one is causing problems. Some network applications require static addressing.

If you want to consider dynamic addressing because it is easier to set up, CIT advises that you limit the range for dynamically assigned addresses to start at 21, thus leaving 6-20 reserved for hosts that need fixed addresses because they will offer IP services. If you think you will have more than 15 IP server hosts, you should raise the start point for dynamically assigned addresses.

Host Registration

Network administrators are required to register all devices on their networks (desktop workstations, servers, printers, etc.). See How to Comply with Network Registration Policy for more information.

IPv4 Global Address Blocks Owned and Managed by CornellĀ 

Cornell owns and manages the following publicly routable IPv4 global address blocks:

  • 128.84.0.0/16
  • 128.253.0.0/16
  • 132.236.0.0/16
  • 192.35.82.0/24
  • 192.122.235.0/24
  • 192.122.236.0/24
.