Visit to a Far-Flung Subnet
In which Mole addresses issuing IP addresses to clients all over the place.
I’ve learned that DHCP uses broadcast protocol and will
not be relayed through a router. We have around 43 locations
spread out in the U.S. and have Windows NT 4.0 and Bay
Networks routers for routing between the locations. The
issue is that we need our NT server at our corporate site
to issue IP addresses to our clients at all locations
with different subnets. Need your advice.
Thinking out loud, Mole can come up with a couple of ways
for your clients to obtain an IP address. The first way
is to get the IP address from your router. For the sake
of discussion, Mole is going to assume that your Bay Networks
router is one of the Nautica 4000 series. The Nautica
4000 can be configured to function as a local DHCP server
for workstations connected directly to the local Ethernet
segment. This is fine if you’ve got only one segment,
but you need one central NT server to hand out IP addresses
to clients all around the U.S. (Letting several servers
or routers on your network think that they’re the only
ones that matter and handing out IP addresses willy-nilly
will soon cause problems with duplicate IP addresses on
the network. And we know we can’t have that, right?)
Now, DHCP is an extension of the initial Bootstrap Protocol
or BOOTP, which assigned IP addresses from a database
of physical addresses matching IP addresses. What you
need to do is configure the routers to be BootP (Bootstrap
Protocol) Relay Agents, called BootP Relay Mode on Bay
Network routers. DHCP gives a framework for passing configuration
information to IP hosts connecting to a network, as defined
under RFC1541. BootP Relay Mode passes IP address requests
and delivers host-specific configuration parameters from
a DHCP server to a DHCP client.
A general overview of Bay Network’s Nautica 4000 Router
can be read from Bay Network’s Nautica 4000 Web page at
So, to summarize, if you want to implement DHCP, and
clients are unable to obtain IP addresses, and you have
a multiple subnet configuration, then the following information
applies to you.
A DHCP server can provide IP addresses to client computers
spanning multiple subnets if the router that separates
them can act as an RFC 1542 (BOOTP) relay agent. To proceed:
- Configure a BOOTP/DHCP Relay Agent on the client
segment. This can be the router itself or an NT computer
running the DHCP relay service.
- Configure a DHCP scope to match the network address
on the other side of the router where the clients are.
Make sure the subnet mask is correct. Don’t configure
a default gateway on the DHCP server’s NIC such that
it’s the same address as that of the router supporting
the subnet where the clients are. Don’t include that
subnet’s scope in any superscope configured on the DHCP
server’s LAN segment.
- Make sure there’s only one logical route between
the server and the remote clients.
For more information, read this KB article: “How to Configure
Microsoft DHCP Server for BOOTP Clients (Q174765).
And if that doesn’t satisfy your information hunger,
Mole suggests querying the Knowledge Base using the query
“bootp”. A good backgrounder is a TechNet article, “Managing
TCP/IP Addresses on Your Network with DHCP,” located at
You’ve got it knocked, man.