ISP Subnet - how does it work?

jonnytabpni

Hi Everyone,

In the past I have setup public IPs on a network as follows:

The DSL ISP will give us a public subnet (/29). To use these IPs, this is how I configured the network:

The modem/router has NAT switched off
One IP from the subnet is assigned to the WAN side of the DSL modem/router (This is done automatically via PPPoA)
One IP from the subnet is assigned to the LAN side of the DSL modem/router (I configure this myself, and usually just pick the next one after the WAN IP)
The rest are used for the hosts on the LAN

The above works, and it's what I've always done. However there is one thing that is bugging me: How can the WAN and LAN sides of the modem/router have IPs in the same subnet? In the way of traditional routing, each interface is on a different subnet.

I'm very confused about this

Thanks

Jonny

novacoresystems

No this is pretty much the wrong way of doing things for what you are trying to do. Let's say for example you have 5 static IPs:

One WAN IP out of the 5 will be assigned to the WAN interface of the router
private IP scheme assigned to LAN interface. For example: 192.168.1.1
Additional IPs for the WAN can either be assigned by adding them as an additional WAN interface on the router or using different routers and plugging them into the same modem in an available port or by using a switch.

From what it sounded like you were doing was you were giving internal workstations/servers public IPs.. why bother even using a router then? All internal devices should have private IP addressing such as..

10.x.x.x
172.x.x.x
192.x.x.x