What Is a DHCP Client ID?

By Stephen Byron Cooper

The DHCP Client ID enables a computer to participate in Dynamic Host Configuration Protocol procedures. DHCP is a network administration tool that allocates an IP address to a computer over the network before the computer's operating system has booted. This is particularly necessary in networks where the boot program is available from a network location and not installed on the computer.

IP Addresses

IP addresses are defined by the Internet Protocol. Computers need an IP address to contact other computers across the Internet. However, the stock of available IP addresses for new installations has been running out for a number of years. Internet service providers and network administrators use a method called Dynamic IP addressing to slow down the exhaustion of available IP addresses. Dynamic IP addressing involves a pool of IP addresses available to a network where the number of computers on that network is greater than the number of IP addresses in the pool. A computer needs the same IP address for the duration of its connection to the Internet but it does not need the same IP address forever. Therefore, network administrators only give those computers turned on an IP address. If a network includes 100 computers, but only 70 percent of these are ever active at any one time, the network can get away with only having 70 IP addresses without impairing Internet access.

Network Adapter

The DHCP procedures are resident on the processor on the network adapter of each computer. As soon as the computer powers up, the network adapter's processor contacts the DHCP server on the network to request an IP address. The network adapter is also known as the Ethernet card or the LAN card. It is responsible for converting data from a computer into the format required to travel over the cable. It performs "Media Access Control," which is the task of listening for silence on the network cable and applying a signal when one is detected.

MAC Address

The DHCP Client ID that the network adapter sends to the DHCP server is its MAC address. The MAC address ("Media Access Control") is the physical address of a computer, and is the unique serial number burned into every network adapter. Network gateways that stand at the access point between the local network and the Internet maintain a lookup table that cross references between the MAC addresses and IP addresses on the network.

Other Client IDs

Because the network needs to maintain a cross reference between MAC addresses and IP addresses it is logical that the client sends its MAC address as its DHCP client ID, and this is the default on all DHCP systems. However, network administrators can override this default and enter a custom series of DHCP client IDs on each computer.