Network Topology to Avoid

In some IP network environments, several local area networks (LANs) share the same network hardware media. The networks may use multiple network hardware interfaces or multiple logical interfaces. DHCP does not work well in this kind of shared media network. When multiple LANs run across the same physical network, a DHCP client's request arrives on all network hardware interfaces. This effect makes the client appear to be attached to all of the IP networks simultaneously.

DHCP must be able to determine the address of a client's network in order to assign an appropriate IP address to the client. If more than one network is present on the hardware media, the server cannot determine the client's network. The server cannot assign an IP address without knowing the network number.

You can use DHCP on only one of the networks. If one network does not suit your DHCP needs, you must reconfigure the networks. You should consider the following suggestions:

• Use a variable length subnet mask (VLSM) on your subnets to make better use of the IP address space you have. You may not need to run multiple networks on the same physical network. See the netmasks(4) man page for information about implementing variable length subnetting. For more detailed information about Classless Inter-Domain Routing (CIDR) and VLSM, see http://www.ietf.org/rfc/rfc1519.txt.

• Configure the ports on your switches to assign devices to different physical LANs. This technique preserves the mapping of one LAN to one IP network, required for Solaris DHCP. See the documentation for the switch for information about port configuration.