WAN IP Address Help

I have a cable connection to the internet and having difficulty understanding what is happening with my WAN IP address. Please note I'm not using the correct IPs here.

When I connect my Cisco PIX 515 firewall to the Cable Modem (via the outside interface) the IP address reported for the outside interface is 82.39.xx.xx.

However if I replace the PIX firewall with a Linksys Cable/DSL Router connected to the cable modem the WAN IP is reported as 82.39.yy.yy.

Where xx.xx and yy.yy are different.

I thought that the outside interface of the PIX would pickup the same IP address as reported by the Linksys router, I.e. the WAN IP address.

What am I missing here?


Reply to
Cisco Newbie
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Just to confirm the 2 setups I have at present:

PIX Setup (internal LAN)(Cisco 2600 Switch)(Cisco PIX 515)(Cable Modem)(ISP)

192.168.1.n/24 82.39.xx.xx (Outside Interface IP Address)

LinkSys Setup (internal LAN)(LinkSys Cable Router BEFSR81)(Cable Modem)(ISP)

192.168.1.n/24 82.39.yy.yy (WAN IP Address)


Reply to
Cisco Newbie


This is because the DHCP process reserves IP adresses per MAC-address. Your linksys has an other mac-address as the PIX. This causes the DHCP process at the provider to reserve 2 ip adresses (untill either lease expires). If this is a problem, set the linksys to use the same MAC adres as the PIX outside interface. A change of hostname may also cause this behaviour. You can set the linksys to propagate the same hostname as the pix.


Reply to
Erik Tamminga


a lot depends on how your cable co sets up the network.

it sounds like they use DHCP to allocate IP addresses, and since DHCP uses MAC addresses to track assignments, the 2 devices get different IP.

i have a home cable connection (NTL in the UK) - i get a single IP. if i swap devices, then i have to reboot the cable modem to allow them to use the link, and each device has to be registered on their web site against my account. NTL limit each subscriber to a single IP address at any 1 time -sounds like your provider is a bit more relaxed (or their security isnt set up the same).

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