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DHCP - What Is It?

*   What is DHCP?

DHCP stands for Dynamic Host Configuration Protocol. DHCP's purpose is to enable individual computers on an IP network to extract their configurations from a server (the 'DHCP server'); in particular, servers that have no exact information about the individual computers until they request the information. When using the Internet's set of protocols (TCP/IP), in order for a computer system to communicate to another computer system, it needs a unique IP address. Without DHCP, the IP address must be assigned individually by ITS and then entered manually at each computer system. With DHCP, the system automatically obtains an IP address from the server during the boot-up process, requiring no intervention on the part of either ITS or the user (once the user configures their computer to use DHCP). [1] [2]

*   Why Use DHCP?

DHCP is highly recommended for several reasons:
*   Eliminates the need for manual client configuration
Manual configuration requires the careful input of a unique IP address, subnet mask, default router address and a DNS (Domain Name Server) address. In an ideal world, manually assigning client addresses should be relatively straight forward and error free. Unfortunately, we do not live in an ideal world; computers are frequently moved and new systems get added to a network. In addition, problems can occur when manually setting up your client machines.
*   Efficient utilization of IP Address space
Each computer gets its configuration from a "pool" of available numbers automatically for a specific time period (called a leasing period), meaning no wasted numbers. When a computer has finished with the address, it is released for another computer to use.

Static assignment of IP addresses ultimately results in poor utilization of our address space, whereas dynamic assignment virtually guarantees that underutilization won't occur. Static assignment doesn't offer any automated mechanism for returning unused or abandoned IP addresses. [3]

*   Ease of changing network parameters
Major network resource changes requires only the DHCP server be updated with the new information, rather than every system. [2]
*   Host mobility is enabled
DHCP provides the capability for a client to connect to any subnet that has DHCP without changing the setup. Thus, users with laptops can easily rove campus without having to ever modify their network configuration if using DHCP. [3]
*   Immediate and automatic address assignment
IP addresses are assigned by the DHCP server automatically, without the need for manual intervention. The latency of requesting a Static IP to be assigned is eliminated, simplifying the work for users, network administrators, and ITS. [3]

*   Client Configuration for DHCP

Client configuration can be found at

*   DHCP DNS Entries

DNS entries corresponding to dynamically assigned addresses are of the form:

[bldg] = Building where the network is located
[subnet] = Subnet on which the DHCP service is provided
[node] = Individual node on the subnet.

For example, if located in the Computing Center on the subnet, an example DNS entry would be: comp113-###-dhcp.

ITS does not support mapping specific hostnames to DHCP IPs through the dynamic IP capabilities of the DHCP server.
Hosts that require a specific hostname MUST use a static IP address [See Below]. [3]

*   Static IP Addresses

Some systems will require that a static IP address be assigned.
These systems may include:
*   Unix Workstations
*   Networked Printers
*   WWW, FTP, Mail Servers, etc.
*   NFS Servers
*   Backup Clients/Servers

To assign an IP for these devices, go to /ipapp/ipapp.cgi.

*   How DHCP Works (Technical Details)

DHCP assigns a number automatically based on a defined range of numbers (i.e., a scope) that belongs to a network. DHCP does not need DNS (Domain Name Service) to function, although many IP services (such as TCP wrappers) require that there be a DNS entry.

DHCP assigns a TCP/IP address when a system is started. It works like this:

  1. A user turns on a machine with a DHCP client.
  2. The machine goes to the router and looks for a DHCP helper address.
  3. The router directs the machine to the correct DHCP server.
  4. The client sends a DHCP REQUEST packet.
  5. The server send a DHCP OFFER packet.
  6. The client sends a DHCP ACK packet.
  7. The server assigns an IP number according to the scope range defined on the server.

For a technical review of DHCP standards, see RFCs 2131 and 2132. [4]

*   Sources


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Last Updated:  Friday, March 08, 2013  by  Hostmaster