The University of Texas at Austin

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IP Address Management

What is an IP Address?

An Internet Protocol (IP) address is an identifier for a computer or device on a Transmission Control Protocol/Internet Protocol (TCP/IP) network. TCP/IP is the suite of protocols used to connect hosts on the Internet and to transmit data over networks. While many people rely on domain names to find specific computers on a network, TCP/IP networks do not. Instead, networks using TCP/IP route packets based on the IP address of the destination; DNS service is required to resolve the easily remembered domain names to the network-required IP addresses.

The format of an IP address is a 32-bit numeric address written as four decimal numbers (called octets or dotted-decimal quads) separated by periods; each number can be written as zero to 255 (e.g., 0.0.0.0 to 255.255.255.255). An IP address has two components, the network address and the host address. Determining the network component versus the host component depends on the class or CIDR allocation of the IP-address assignment.

How IP Addresses are Assigned

Traditionally, one of the Regional Internet Registries (RIR) — ARIN, APNIC, and RIPE Network Coordination Centre — assigned Internet addresses to organizations from one of three classes: Class A, Class B, and Class C. The number of unassigned Internet numbers has been rapidly dwindling for a number of years, however, and several solutions over the years tried to deal with this problem. First, subnetting allowed a class to be further distributed into subnetworks. This further divided the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet. Now, subnetting typically refers to the allocation of CIDR blocks.

CIDR (Classless Inter-Domain Routing), the solution in place today, was proposed in the early 1990s to allow ISPs and other organizations (Local Internet Registries) to allocate smaller blocks of IP addresses to their users. This new classless system of assigning IP addresses allows for a more efficient allocation of IP addresses. A CIDR IP address looks like a normal IP address except that it ends with a slash followed by a number, called the IP prefix. The IP prefix specifies how many addresses are covered by the CIDR address, with lower numbers covering more addresses.

CIDR Allocation at the University

UTnic uses CIDR block allocation to assign IP addresses to departments depending on the anticipated number of hosts within each network. The department network administrator then assigns specific IP addresses to specific devices out of these blocks.

CIDR Blocks Assigned at the University, with Corresponding Subnet Masks.
IP Prefix Number of Addresses Subnet Mask
/27 30 255.255.255.224
/26 62 255.255.255.192
/25 126 255.255.255.128
/24 254 (former Class C) 255.255.255.0

DHCP Assignments at the University

DHCP (Dynamic Host Configuration Protocol) dynamically assigns the information necessary for a device to participate on a TCP/IP based network (e.g., IP address, netmask, default gateway). Many members of the university community use DHCP for their portable computers or PDAs: DHCP makes it possible to move from building to building without changing any network settings. In addition, Resnet, and the Public Network access uses DHCP to assign IP addresses to users.

Dynamic addressing simplifies network administration because the local administrator does not need to assign specific IP addresses to specific devices; instead, one of the many IP addresses allocated for DHCP use is assigned when a user connects to UTnet.

Further Reading

Last updated November 16, 2012 @ 3:59 pm

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