Network Topologies That Limit Effective Use of DHCP/BOOTP

DHCP and BOOTP clients initially have no information about the local IP network, and therefore use the 0.0.0.0 (the default network address) for their IP address. The clients send their DHCP or BOOTP requests to the 255.255.255.255 IP address (broadcast address) which will be received by all IP devices attached to the local IP network.

DHCP and BOOTP servers determine the client's IP network attachment based on following factors:

1. Which network hardware interface the DHCP or BOOTP request was received on?

2. Was the DHCP or BOOTP request received from a BOOTP relay agent?

   BOOTP relay agents insert the IP address of their network hardware interface that is attached to the same IP network as the DHCP or BOOTP client. The absence of this IP address signals to the server that the client is on a directly-attached IP network. The presence of this IP address indicates that the client is attached to an IP network remote from the server and that the server is to send responses back to the client using the BOOTP relay agent's IP address.

3. Is the IP network the client is attached to subnetted?

   The servers, using the IP address as a key, consults the contents of the netmasks table (which contains subnet mask information) using either:

   • The IP address of the server's network hardware interface if the client is on a directly-attached IP network (indicated by an IP address of 0.0.0.0 in the packet's relay address field)

   • The IP address specified if a BOOTP relay agent specified its IP address in the client's request

   (See netmasks(4) for information on netmasks.)

This procedure for determining the client's IP network attachment is only effective if there is only one IP network present on the network hardware media (for example, Ethernet). DHCP does not work well in IP network environments where, either through the use of multiple network hardware interfaces or multiple logical interfaces, there is more than one IP network sharing the same network hardware media. In this case, a DHCP client's request appears on all network hardware interfaces, making the client "appear" to be attached to all of the IP networks simultaneously. Since DHCP servers dynamically allocate IP addresses to requesting clients, the server is unable to determine which IP address to allocate to the client. DHCP clients, attempting to validate IP addresses they presently hold, will be seen to originate on all the logical IP networks, not just the network the client has been assigned to.

Such network topologies should be avoided, either though the use of Variable Length Subnet Masks (VLSM) to preserve the one-to-one mapping of IP network to network hardware media through more efficient subnetting, or by configuring the DHCP or BOOTP service to serve just one of the logical IP networks. See netmasks(4) for additional information.