one external IP to multiple internal IPs? Ideas?

I am looking for ideas.

We have an As/400 that we are using for web serving.  Our network group has
a router set up to route requests to it.

This server will be serving multiple companies for customer service.  What I
would like to do ideally is set up an instance for each company (using
BindSpecific) so that each of them can run in seperate jobs.  Also, logging
seperately is another reason I want to do this.

What we plan on doing is making a subdomain for the companies that will
route these requests to the AS/400.  For example, company1.com will have a
subdomain of cso.company1.com that will be routed a particular external IP
address, then routed to the AS/400.

Our network guys tell me they can only route IPs on a one to one basis, so
to run multiple servers I would need an external IP for each company.

So, here's what I want..

URL                 Ext. IP      AS/400 IP
cso.company1.com -> 218.1.1.2 -> 172.25.6.1
cso.company2.com -> 218.1.1.2 -> 172.25.6.2
etc.. 
etc..

Does anyone else have any better ideas?  I know I can use virtual hosts and
one IP address, but like I said, I'd rather keep them seperate.

Brad> 
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