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I have a networking concept that I'm having difficulty understanding where I hope someone can point me in the right direction. If I have the following configuration for a router, and it receives a packet with a source address of 192.168.1.35and a destination address of 192.168.1.85, what interface will the router forward the packet from?

interface fastethernet 0/1
no shutdown
interface fastethernet 0/1.1
encapsulation dot1q 1
ip address 192.168.1.62 255.255.255.224
interface fastethernet 0/1.2
encapsulation dot1q 2
ip address 192.168.1.94 255.255.255.224
interface fastethernet 0/1.3
encapsulation dot1q 3
ip address 192.168.1.126 255.255.255.224

So my understanding is that each subnet is a /27 subnet, which have 30 available hosts (considering network and broadcast addresses).

I believe that the address range for the first subnet will therefore be 192.168.1.62-92 and therefore the packet will be forwarded from fe 0/1.1. However, in a practice exam, I'm told the answer is in actual fact fe 0/1.2. Could anyone tell me why this is? Or point me to a resource that explains this. As I'm not sure how a packet could be forwarded from this port when I assume the range would be 192.168.1.94-124.

Despite this, I may be assuming that the router IP address is allocated for the first host in the subnet. If the router was the last host in the subnet then perhaps this could be understood where:

192.168.1.63 (Network Address)
192.168.1.64 (1st Addressable Host)
192.168.1.94 (Router (Default GW))
192.168.1.95 (Broadcast)

I think I may have worked this out while I wrote this question but I would appreciate confirmation.

  • 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 could provide and accept your own answer. – Ron Maupin Apr 1 '18 at 20:31
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So basically a packet is traversing your router with: Source IP Address:192.168.1.35 Destination IP Address: 192.168.1.85

Your device does a lookup and verifies that to reach x.x.1.85 is via:

interface fastethernet 0/1.2

encapsulation dot1q 2

ip address 192.168.1.94 255.255.255.224

See the Host Address Range below:

enter image description here

The address you configured on your interface matches the last available address in the range.

  • 2
    @user3592021 you should accept the answer. :-) – user36472 Feb 4 '18 at 23:11

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