Dynamic Host Configuration Protocol Requirements :
DHCP is not required to support an ISA Server infrastructure, but it is highly recommended to simplify network management. Even on relatively small networks of 250 or fewer computers, you will benefit from reduced administrative effort by configuring a DHCP server on your network. The advantage of using DHCP is that it can provide the IP configuration for all the client computers on your network automatically. This can make your ISA Server deployment much more efficient. For example, if you need to reconfigure the default gateway for all your client computers to point to the new ISA Server computer or to a new DNS server for Internet name resolution, you can just change the scope setting on the DHCP server and all the clients will be reconfigured automatically.
DHCP is also used to support VPN remote access connections to ISA Server. By default, ISA Server will use DHCP to assign IP addresses to all VPN clients. When you enable remote VPN client access on ISA Server, it will obtain a set of IP addresses from the DHCP server and assign the IP address to the VPN clients. By default, ISA Server 2004 will also assign DNS or WINS server addresses based on the DHCP scope information.
MCP 70-350 : Installing ISA Server 2004
