0

I have the following network setup:

PC -> Router (local network) -> Modem (123.123.123.123) -> Internet

The LAN DNS is configured (via dhcp):

dns 1: ISP dns 1

dns 2: ISP dns 2

dns 3: Router

My Public IP (example):

123.123.123.123

When I visit http://123.123.123.123 from my local (router's) network, it resolves to my router login page. Strange.

If I visit http://123.123.123.123 from an external network it doesn't resolve anything (as to be expected).

I ran a tracert from my local network and I get 1 hop that just resolves to my ISP.

> tracert 123.123.123.123

Tracing route to aaa-123-123-123-123.isp.com [123.123.123.123] over a maximum of 30 hops:

  1     1 ms     1 ms     1 ms  aaa-123-123-123-123.isp.com [123.123.123.123]

When I run a tracert from an external network I get a lot of hops (as expected).

1     7 ms     1 ms     1 ms  xxx.xxx.xxx.xxx
2   115 ms    41 ms    35 ms  xxx.xxx.xxx.xxx
3   169 ms    41 ms    37 ms  xxx.xxx.xxx.xxx
4    76 ms    75 ms    43 ms  xxx.xxx.xxx.xxx
...

How is my request for 123.123.123.123 (an external ip address) being routed directly to my router on the local network and why?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Feb 19 '18 at 18:27
2

Your router has two IP addresses, one inside the LAN, one outside (WAN). From the router's perspective, both addresses are local. The outside, public IP isn't your ISP, it's an IP address temporarily issued to your router.

http://123.123.123.123 from LAN probably works because from your PC's perspective it is remote, so it get's routed to your default gateway, the router. Your router sees this IP address pointing to itself and treats it like the LAN IP or the loopback. This is sometimes called 'hairpin' routing and may be treated different than traffic from WAN.

  • Could also be a flavor of proxy ARP on the gateway device. Many devices (Cisco ASA is a prime example) support proxy ARP by default for any IP address the device believes it has control of. – YLearn Sep 4 '17 at 14:03
0

Some of SOHO routers have set as default forwarding to their home page when there is no internet access from WAN side. Check, does your router have configured correctly WAN ip address, gateway and mask. Also check do you have default route configured. It should look like

Network: 0.0.0.0/0 gateway: 123.123.123.123

or simmilar.

0

Your default gateway will be your router, when you try and access an external address it will send the traffic to your default gateway. When your traffic hits the router it will see the address as itself and see that you are coming from the LAN network and allow the traffic. Further to this the traceroute will only show one hop as it effectively only hits one router.

When you try and access from the outside there is most probably a "Remote Access" checkbox unchecked (which it should be) denying management access from outside your local (LAN) network.

0

First note that the names you see in traceroute come from a process called "reverse DNS", traceroute translates the IP into a special hostname and then asks the DNS system for a "ptr" record mapping that to an actual hostname. The hostname that is returned is chosen by whoever controls the reverse dns for the IP block, typically the ISP. For residential connections it will usually be set to something containing the ISPs name and part or all of the IP address.

The fact that you get a reverse DNS response with your ISPs name in it does not mean the traffic actually went to your ISP.


Your router has two IP addresses, an internal one for your local network and an external one on the outside.

Your client looks up 123.123.123.123 in it's routing table, decides it doesn't have a better match and sends the packet to it's default gateway which is your router.

When the router receives the packet it looks up to see whether the destination IP is one of it's IPs. It is so it processes the packet locally.

Why do you see the admin interface when accessing the public IP from inside but not from outside? Most likely the answer is your router has firewall rules for local traffic which are based on the incoming interface. So the request coming in from outside gets blocked but the request coming from inside gets let through.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.