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I am confused by the following:

C:\>ipconfig
....
        Connection-specific DNS Suffix  . : some address here
        IP Address. . . . . . . . . . . . : 10.77.121.54
        Subnet Mask . . . . . . . . . . . : 255.255.255.0
        Default Gateway . . . . . . . . . : 10.77.121.1

From the above, the gateway for my pc is 10.77.121.1. But when I use the tracert, the first IP address is different as you can see from the following. It is 10.77.121.3. Why?

C:\tracert 10.75.89.100

Tracing route to 10.75.89.100
over a maximum of 30 hops:

  1    29 ms    <1 ms    <1 ms  10.77.121.3
  2    <1 ms    <1 ms    <1 ms  10.77.0.34
  3     1 ms    <1 ms    <1 ms  10.77.0.45
  4    <1 ms    <1 ms    <1 ms  10.75.89.100
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4 Answers

It looks like the gateway you're using may be using VRRP or HSRP. 10.77.121.1 is a virtual/logical address, which you use as your gateway, but when you trace route one of the physical routers responds. (10.77.121.3)

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Your default gateway isn't always the next hop. In fact it is generally the router of last resort. Your computer will parse the routing table and will only use the default gateway if no other more specific routes match. So do a route print and see what you find there.

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From the above, the gateway for my pc is 10.77.121.1. But when I use the tracert, the first IP address is different as you can see from the following. It is 10.77.121.3. Why?

It is rare, but perhaps 10.77.121.1 no longer has a connection to the next hop, but it still has forwarding enabled. If this is the case, with some operating systems, when you attempt to send a packet you will contact 10.77.121.1, but 10.77.121.1 will return an ICMP redirect with 10.77.121.3 as the address your system be using as a gateway. Since your packet is not actually being forwarded by 10.77.121.1 it wouldn't show up in a trace route.

A quick capture with your favorite sniffer on your client would confirm that this is happening.

ICMP redirects are mostly discouraged these days for security reasons, so having a setup like this is uncommon.

Network Diagram

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It could be @MikeR's answer. It could also be that your machine has a route to the 10.75.89.100 address that doesn't go through the default gateway, but instead goes through 10.77.121.3. Check your host's routing table to see if that's the case.

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