DHCP - Client ID

The DHCP (Dynamic Host Configuration Protocol) client ID is a unique identifier that a DHCP client can use when communicating with a DHCP server. This identifier is used to ensure that each client on a network is uniquely identified, which is particularly important in managing the assignment of IPv4 and IPv6 addresses and other network configurations.

In most implementations where Ethernet is used as the Layer 2 technology, the client ID that is used is the MAC address. A DHCP server, within each binding, will identify the client using its unique MAC address.

However, the client ID can take other forms as well. It is the client that decides what client ID to use whenever responding to the DHCP server.

In a standard DHCP server setup, the server usually doesn’t care about the client ID’s format or content as long as it is unique. It will simply provide an IP address and other network configuration details based on the availability of IP addresses in its pool. The client ID is stored in the binding, and is used mainly for identifying the uniqueness of the request.

Cisco devices that act as DHCP clients will by default use a proprietary client ID format.