How can the ISP's default gateway be in a different subnet and still work with my public address?

[dlais1987]

How is it possible that the default gateway on 24.205.84.1 works with an address 24.205.86.91 /30 on the wan of my home router?

[Georg Pauwen · Accepted solution]

Hello,

actually, two interfaces do not need to be on the same subnet in order to talk to each other. The IP addresses can be completely different.

As long as one interface ARPs for the IP address of the next hop, that next hop can be anything. If you have access to a lab, try the setup below:

Home Router

interface GigabitEthernet0/0

ip address 24.205.84.1 255.255.255.252

!

ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/0

ISP Router

interface GigabitEthernet0/0

ip address  24.205.86.91255.255.255.248

!

ip route 0.0.0.0 0.0.0.0 GigabitEthernet0/0

Both routers will be able to ping each other.

[dlais1987]

Thank you George! Preparing for the ccna, I was always told that the default gateway on the physical or sub interface has to be in the same subnet while the dns and dhcp can be in different subnets via relay etc These ip I got of the gui of my routers page but I still dont understand how a different subnet could be a default gateway in this instance even with the default route or is this because I am wrongly thinking about my router acting like a pc and this default gateway is actually the routers default route as well as default gateway out to the isps subnet?

[Georg Pauwen]

Hello,

just to be sure there is no misunderstanding:

Your PC --> Default Gatway --> WAN Interface (Local) --> WAN Interface (Remote ISP)

Whcih subnet are we talking about, the one between your PC and the default gateway, or the one between the two WAN interfaces ?

[dlais1987]

WAN Interface (Local) --> WAN Interface (Remote ISP)

[Georg Pauwen · Accepted solution]

Helllo,

ok, we are on the same page, and the explanations provided are valid. In real life, both connected interface are actually on the same subnet pretty much most of the time. ISPs however are short on IP addresses, they usually don't want to give you a full subnet, so you might get a host address, or an address in a different subnet.

[paul driver · Accepted solution]

Hello
In this instance your wan rtr will sent a arp request for its default-gateway mac address via its connected wan interface which by the way both rtrs connected interface could be in the correct /30 address scope of each other and then the adjacent rtr in this case would see it has the ip address on another one of its interfaces and then would unicast a arp reply back to you wan rtr

FYI - your wan rtr ip address doesn’t look correct for a /30 it’s either .89 or .90

[dlais1987]

Thank you Paul for your reply I am still a little confused as to how my router can have the next hop be outside of its subnet. 

[paul driver]

Hello
It could be how i describe it earlier or your wan rtr ip addressing is part of a larger subnet that the isp distributes to its customers but due to historical configuration the default gateway is incorrect and proxy arp is saving the day.

I would suggest to check with your ISP for confirmation.

[carl_townshend]

Hi Paul,

The config you described above would not work without proxy arp right ?