Question regarding multiple external IP for one net connection

[cross6]

here's a diagram of our network

http://img163.imageshack.us/img163/1949/drawing19ax.gif

Now ignore the dsl, it only switches over for net access if the cable goes down.


OK, we have about 3 static EXTERNAL IP's for our net connection on the cable.

They are listed as:

VPN
GATEWAY
MX RECORD

The mx record external IP hits our exchange 2k server inside our network

obviously the gateway is what pages like whatismyip.net say our external is

vpn is the ip we hit with our vpn client

But all three of these ip's are hitting our modem - how does this work?

How does the modem know which ip's go where?

Never heard of multiple external static ip's on a cable modem

 

[n0cmonkey]

Does the cable modem have an IP address for you guys, or does it just sit there and bridge stuff?

 

[cross6]

you mean the internal ip of the cable modem?

yes it has an ip to the cisco router

 

[n0cmonkey]

Originally posted by: cross6
you mean the internal ip of the cable modem?

yes it has an ip to the cisco router

Huh?

 

[cross6]

Originally posted by: n0cmonkey

Originally posted by: cross6
you mean the internal ip of the cable modem?

yes it has an ip to the cisco router

Huh?

I have no idea what you are asking then

how hard is it to understand, we have several static external ip's coming to one normal plain surfboard cable modem

 

[n0cmonkey]

Originally posted by: cross6

Originally posted by: n0cmonkey

Originally posted by: cross6
you mean the internal ip of the cable modem?

yes it has an ip to the cisco router

Huh?

I have no idea what you are asking then

how hard is it to understand, we have several static external ip's coming to one normal plain surfboard cable modem

They are bound to the cable modem's interfaces, or does the cable modem just bridge the networks without IPs?

If the surfboard doesn't have an ip address that is used as the gateway or anything, it shouldn't matter how many IPs are behind it. My mom has DSL and we had 4-5 computers, each with unique IPs behind the little toshiba DSL device. Worked just fine.

If the IP addresses are assigned to the cable modem (this is what I was asking basically), then someone has configured it to do 1:1 NATs for the VPN and MX hosts. Not hard to setup with a decent system.

 

[cross6]

If the surfboard doesn't have an ip address that is used as the gateway or anything, it shouldn't matter how many IPs are behind it. My mom has DSL and we had 4-5 computers, each with unique IPs behind the little toshiba DSL device. Worked just fine.

Dude, external public ip's. Not internal.

 

[spidey07]

sounds like the PIX is doing the address translations for you. That is the normal application of a firewall.

As far as multiple IPs that is just normal IP routing.

 

[n0cmonkey]

Originally posted by: cross6

If the surfboard doesn't have an ip address that is used as the gateway or anything, it shouldn't matter how many IPs are behind it. My mom has DSL and we had 4-5 computers, each with unique IPs behind the little toshiba DSL device. Worked just fine.

Dude, external public ip's. Not internal.

Dude, that's what I meant. We used 4-5 external, public, non-RFC 1918 compliant IP addresses assigned to us by the Verizon DHCP servers.

 

[n0cmonkey]

EDIT: DP

 

[dnoyeb]

Dude, they all do that. I can order a 2nd IP address from comcast now. I won't need a 2nd modem. Right now my modem gives out just 1 IP address through DHCP, but its capable of giving out 32 I believe. Its a motorola.

based on observation it works like this

1. modem gives out the 3 IP addresses in response to 3 DHCP requests. It will only give as many as the cable company tells it it can give out. After this the cable modem becomes transparent totally and forwards _everything_ it can see. (thats what I think)
2. when ARP request comes to cable modem they are forwarded to the LAN side. So your LAN devices respond to ARP request. So your LAN items are actually address by their MAC addresses. The cable modem is like a switch/bridge not a router.

Also this requires special understanding by your PIX devices. Many broadband routers can only handle a single WAN side IP address. My Zyxel P334WT can handle 2-3. Of course a Cisco PIX is probably a proper router.