Public IP different than ISP gateway

[mefasherman54936]

Hey,

I have a situation in my home router which puzzles me.

My external IP is not on the same network as my gateway.

For example:

Public IP: 1.1.1.1

Gateway: 3.3.3.3

How is that possible?

[balaji.bandi]

its ISP / SP provider network, do not worry much as long as your network is working and you able to browse the internet all working.

[paul driver]

Hello

How is your home network physically setup?

[Georg Pauwen]

Hello,

as long as proxy-arp is enabled (it is enabled by default on Cisco routers), directly connected routers don't actually have to be configured with IP addresses in the same subnet. So the below works fine:

HOME_ROUTER

interface GigabitEthernet0/0

description Link to ISP_ROUTER

ip address 1.1.1.1 255.255.255.0

!

ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/0

ISP_ROUTER

interface GigabitEthernet0/0

description Link to HOME_ROUTER

ip address 3.3.3.3 255.255.255.0

!

ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/0

[mefasherman54936]

Thank you community for all your replies.

I'm studying for the CCNA exam, and this situation is very interesting to me. 

I tried the configuration "ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/0" in Packet Tracer, it did not work.

The routers did not respond to ping.

Thanks for the suggestion though, I did not realize you can map a default route to an interface, and not an IP. 

I have a regular TP-Link router with PPPoE ADSL connection.

How does the ISP even assign this DHCP IP address to the home user?

[Georg Pauwen]

Hello,

Packet Tracer is not the best tool to test with. On real routers, it works. PT is basically just a subset of predefined commands, not a full IOS.

[Giuseppe Larosa · Accepted solution]

Hello @mefasherman54936 ,

>>

have a regular TP-Link router with PPPoE ADSL connection.

How does the ISP even assign this DHCP IP address to the home user?

The PPPoE builds a virtual point to point link over ethernet and the address assigned to the WAN interface of your router is assigned by PPP IPCP process and not by DHCP.

DHCP is used by TP_link router on the LAN side to assign an IP address to your PC.

By the way, the fact that you are using PPPoE can explain also your initial question : the WAN address is assigned by IPCP taken from a broadband  pool and can change at each session / power on. The IP address of the default gateway can be a loopback address on the broadband concentrator that terminates the PPPoE session.

Hope to help

Giuseppe

[mefasherman54936]

Thanks community & Giuseppe.

I'll look up IPCP technology, sounds like something to be familiar with.