Dual-homed with static-IP ISDN and dyn-IP DSL

Dual-homed with static-IP ISDN and dyn-IP DSL

Post by Scott He » Thu, 29 Jan 1998 04:00:00

I'm installing a server at a site which has access to a service
providing a shared T1 for a reasonable price - but you get a dynamic
IP behind a firewall, and we need a static IP to host a server on.  We
can also get Centrex ISDN and a static IP for relatively cheap on a
metered basis.

I'm pondering a hybrid solution.  Use the ISDN to provide the static
IP for all connections initiated from the Internet, while using an
IPMasquerade/NAT router to let the LAN browse the Internet through the
shared T1.  The network would look somewhat like:

   +-------------+ static +--------+ dynamic +-----------+
   | ISDN router +--------+ IPMasq +---------+ shared T1 |
   +-------------+  IP    +---+----+   IP    +-----------+
                     Internal private LAN

All connections initiated from the Internet to the static IP should
communicate through the ISDN.  All connections initiated from the
internal LAN to the Internet should communicate through the shared T1.

Does that make sense?  Anyone done something similar?

I suspect that it's possible, since the IPMasq machine _should_ route
packets from the static IP out the ISDN interface.  What's less clear
is whether I'll have any chance to direct certain other traffic (such
as email) out that interface.  [I mention email because some sites
will block emails who's headers don't appear to mesh with the site the
connection is actually from.]

Perhaps I could put the default route through the ISDN, and use an
http proxy on the IPMasq host to redirect high-volume web surfing
through the shared T1.  I know I can do that with two machines (the
IPMasq with the ISDN as the default gateway, plus a second with the
shared T1 as the default gateway, both on the private LAN).  Can I do
it with one?

Thanks for any insights,

<Favorite unused computer book title: The Compleat Demystified Idiots
  Guide to the Zen of Dummies in a Nutshell in Seven Days, Unleashed>


1. Force Dyn IP to Static IP

I got thinking the other day and was wondering if this was possible...
I am assigned a dynamic IP from my ISP. Would it be possible to some how
force it to use the same IP everytime? (Using DHCP perhaps?). I know that I
run the risk of that IP already being taken, but is it still possible? If
you could somehow find an IP number that is hardly ever used by your ISP,
you could avoid having to pay for a static one.

Thanks for you help,


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