Geek help: static IP addresses

[ciphergoth]

Quick geek help sought with making sure I understand IP routing properly.

Zen have allocated me the static IP address block 82.68.129.72/29. Of these eight addresses, one is the network address, one is the broadcast address.

I've fitted an ADSL card to my main PC, saltationism. So it has two interfaces: an ADSL interface and a LAN one. That's two more addresses taken. So there are four left: I can give up to four more machines on the LAN static IP addresses.

The thing that worries me is that if the subnet for the LAN is 82.68.129.72/29, then can I assign an address from that subnet to the ADSL card? If I do, will my LAN be able to route to that address? If not, what should I do instead?

Comments

[mr-purpleduck.livejournal.com]

It would be best to use NAT on the inside LAN and not use real world addresses, as you would have to use saltationism as a bridge device (i.e. it's being not a router) as the 82.68.129.72/29 subnet can not be on the ADSL interface and the LAN interface at the same time as this would lead to to routing table entries.

I know iptables in Linux has support for bridges, so it would be possible to have this topology. But it would involve using proxy-arp which gives one of those ick feelings.

[ciphergoth.livejournal.com]

I want to assign real IP addresses to at least some machines; it would be convenient to be able to reach them directly from my work, for example. Hmm, actually is it possible to give some machines real IP addreses and others 192.168 style addresses? Hmm...

[kimble.livejournal.com]

Yes, no reason you can't do that (would need an IP alias for eth0 in the 192.168.whatever range and appropriate routing table entries). Packets between the subnets would go via the router, which may be an issue if you care about throughput.

[mr-purpleduck.livejournal.com]

There's nothing to stop to putting different subnets on the same wire, you just have to be very careful to not create any loops. A broadcast storm can easily happen in this situation.

Dose the ADSL interface get assigned a IP address from a different range from your ISP ? In most situations this address would be used to route the subnet though the router and you would not have to use the same subnet on both sides of router.

In Cisco terms you could use ip unnumbered on the ADSL interface and just use the ip address from the eth0 interface, but these days I use more Cisco kit than Linux for network rotes at work.

[ciphergoth.livejournal.com]

No, when I "dial out" the ISP assigns me the last proper address in my range (ie one below the broadcast address).

I don't know what you mean by the Cisco thing...

[mr-purpleduck.livejournal.com]

In which case you will have to do some thing with bridging - I could try to explain it some more in Whitby if you wish. I'm going to be a bit busy between now and heading south.