DHCP Fundamentals

DHCP allows the automatic assignment of network parameters, such as IP addresses, to a number of hosts or devices. Previously, networks were smaller and contained a number of hosts that used static addresses. These networks were easy to manage and administer, but with the proliferation of laptops and network devices, coupled with the increase in user mobility, the administrator could not cope with the demand for short- term address management. DHCP addresses this issue by managing ranges of IP addresses, which can be allocated as required. When an address is no longer needed by a client, it is released and becomes free to be allocated to another client.

Why Use DHCP?

DHCP facilitates the distribution and allocation of IP addresses dynamically, from a central location, the DHCP server. When a client is configured to use DHCP, it automatically obtains a short-term lease of an IP address when it boots up. If the same computer moves to another network, it is automatically assigned a new IP address relevant to that network, without the need for an administrator to manually configure the network interface.

Without the use of DHCP, all IP addresses would have to be manually configured, a task that would quickly become unmanageable in today's large network environments.

What Does a DHCP Server Do?

A DHCP server manages a range of IP addresses. When a request is received from a booting client, known as a DHCPDISCOVER request, the server responds with an offer of an IP address to lease from the server for a finite period of time, known as a DHCPOFFER. The client can receive multiple offers if there are a number of DHCP servers on the network (as long as the servers all manage a different range of IP addresses). The client then selects an offer to receive, which is passed back to the server in the form of a DHCPREQUEST message. The server finally sends a DHCPACK message to confirm the lease. The process that runs on a DHCP server is called in.dhcpd .

What Does a DHCP Client Do?

A DHCP client is responsible for obtaining sufficient information to enable it to configure the network interface so that network communication can occur. The client runs a process called dhcpagent , which listens for a response from a DHCP server, handles the renewing or relinquishing of a lease for an IP address, and configures the network interface when an IP address has been assigned to the client, after checking that no other host on the network is using the assigned IP address.