BT ADSL 1 Static IP NAT. Options (routers)

[ikonia]

a collegue has just purchased a 1 meg BT 1 static IP address ADSL package form BT Broadband.

BT set this up ok, no problems at all, so no complaints so far.

The package shipped with a a BT1801HG router which is a 2wire Router basiclly.

The router is a 4 port router which assigns dhcp address to the 4 ports and masqurades/nats them out of the one static IP address which is assigned to the router.

I was talking this through with him last night, as he wants to really just plug one machine into the router, give it a static internal IP address (as it can't have the internet facing one direct as thats assigned to the router) and then use 2 way NAT so that outgoing traffic is nat'd to the internetfacing IP address and incoming traffic to the internet facing IP address is nat'ed to the internal IP address on the machine he's connected to the router.

Looking at what he's done and what he's saying, he's actually in an awkward situation.

1.) He's using a Linux machine on my recommendation (so my fault) and when he sets a static private IP address range within the routers range, it won't regognise it in the router configuration software. eg: it doesn't show up as connected, although the router is connected to the machine and allowing it access to the internet. The Linux machine does show up if he allows it to be dynamicly assigned an address.

2.) There is no option to configure nat within this router - I've have read posts of users saying they are using nat - yet I have no idea how this can be. The detault nat/masqurading that comes when a dynamic IP address is assigned is out going only, and does not map back to a specific address (eg you can't say telnet to the private address through the internet facing address mapping back to it

I'm interested if anyone on BT with a 1 static IP address has achived this basiclly, a 1 machine configuration 2 way NAT.

I suspect he will need to buy a more advanced router, to which I'd be open to suggestions on.

Comment and advice/discussion appriciated.


3.)

[aidanjt]

Disable DHCP in the router's configuration interface, assign the host a static IP in the same netmask as the router's LAN interface, set the router's configuration interface to assign that host IP in the DMZ.

[ikonia]

I'll see if I can get around to the office he's put it in tonight and grab some screen shots, as I can't see where to do that at all, and the problem is made a little more hard by the fact that if the box sets a static IP it doesn't show up for the router to be viisible for config.

This was how I assumed it would work when we where talking about it, but having a look at it myself there doesn't seem that functionality within the router.

FYI: I'm terrible with DSL style networking products as I don't really use them, so I could way be in the wrong. But the fact the neither my collegue or I can find it, suggests its not there, or well hidden.

[aidanjt]

There's no reason why the router shouldn't be able to communicate with a statically defined host in the same subnet, that would breach god knows how many standards :S

[ikonia]

it can communicate with it, you can access the internet fine, however it just doesn't show up as a device in the router configuration software, which it does if its dhcp assigned an address.

not that this matters at this moment, as I can't see how to setup a single host NAT even if it did show up.

[TheAnimus]

it can communicate with it, you can access the internet fine, however it just doesn't show up as a device in the router configuration software, which it does if its dhcp assigned an address.

not that this matters at this moment, as I can't see how to setup a single host NAT even if it did show up.

The reason it dosen't show up in the software is probably because its not in the DHCP table or DNS or any form of binding. This is normal.

On the linux machine you say you can access the WAN (THE internet).

Can you add that static IP to a DMZ (default mapping zone)?

When you have a NAT gateway, a table is maintianed, and the packets you send get "translated", in other words the source gets fiddled with, all internetworkable IP packets have something like:
ip-addy port
the NAT router replaces ip-addy with its own WAN ip addy, which is differnet to the one for the default gateway setting on the client machine, and oftne changes the port. It maitains a table with the port number it replaced, host ip it replaced, and the new port number it inserted.

That way when a packet is recived, it simply looks at the port number, and decides what it should do to the packet. However problems arise, you cant keep letting that table grow, you have to "prune" entries, this is done based on how many seconds ago the last packet was recived on this port. Those which don't match on the table goto the DMZ ip with the port number unchanged. This is a nice security issue, as some people will hapily run an HTTP server on port 80 on the computer in the DMZ, it all runs great. until another computer on the LAN gets the port 80 of the router translated to it (often by change) then things break, because it wasn't a static route, it was just using the fact its in the DMZ. This is one of the many issues with NAT you need to be aware off, if he intends to run any services on that machine.