Question on Page 330 (near the bottom of the page)

[Pravin]

Hi Todd,

The question on Page 330 asks:

"Based on the output of the next routing table, which interface will a packet with destination address of 10.10.10.14 be forwarded from?

Options are
10.10.10.16/28
10.10.10.8/29
10.10.10.4/30
10.10.10.0/30

I found the question confusing (or is it a trick question? :)). Anyway, I'm wondering, should it have been "which interface will receive a packet with destination address of 10.10.10.14 ?

Cheers.

[enerside]

This is not a trick question, it's a subnetting question.

10.10.10.16/28:
-- Block size of 16, mask of 255.255.255.240
----Host range 10.10.10.17 - 10.10.10.30
------Broadcast of 10.10.10.31

10.10.10.8/29
-- Block size of 8, mask of 255.255.255.248
----Host range 10.10.10.9 - 10.10.10.14
-------Broadcast of 10.10.10.15

10.10.10.4/30
-- Block size of 4, mask of 255.255.255.252
----Host range of 10.10.10.5 - 10.10.10.6
-------Broadcast of 10.10.10.7

10.10.10.0/30
-- Block size of 4, mask of 255.255.255.252
----Host range of 10.10.10.1 - 10.10.10.2
-------Broadcast of 10.10.10.3

So for an IP of 10.10.10.14 we can see it's apart of the 10.10.10.8/29 network, the corresponding interface is your answer. (Sorry, I don't have the book with me to see the entire routing table figure)

Joe

[Pravin]

Thanks for that Joe.

I was confused with the way the question was worded-

ie: which interface will a packet with destination address of 10.10.10.14 be forwarded from?

Would it have been better to ask something along the lines of "which interface will receive a packet with destination address 10.10.10.14?

Cheers.

[enerside]

The router uses it's routing table to determine which interface to send a packet out of based on the destination IP address of the packet. In this case, the router knows that the packet because of it's IP address belongs on the 10.10.10.8/29 network and will send the packet out the interface the network resides, whether its a directly connected network or the router learned that network is available through another router out that interface.

The router has no control over which interface it will receive a certain packet, for example a host with an ip of 10.10.10.12/29 that sits out the fa0/0 interface of the router needs to send a packet to a host at 172.16.156.35. The host knows that it's destination is outside its network so it will send to the default router (gateway). In this case, we know the fa0/0 interface will have an IP address on the 10.10.10.8/29 network but it will be receiving a packet destined for the 172.16.156.35. The router will then use it's routing table to determine if it knows a path. The router sees that it knows the 172.16.156.32/28 network is out the S0/1 interface, which by knowing subnetting we know that 172.16.156.35 sits on the 172.16.156.32/28 network. The router will send the packet out the S0/1 interface.

Basically, the routing table it used to determine where to send, not where to receive. Hope I didn't confuse you more.

Joe

[Pravin]

Thanks Joe. With your explanation, it's now clearer.