A question about 'longest prefix length'?

Beany

Afternoon,

going through the book, im struggling to understand what 'longest prefix length' is in a routing table when a packet's destination route matches a few routes in the routing table?

what does it mean by longest prefix length? got an example in the book but still not sure.

thanks

networker050184

For example you are trying to send packets to a host 192.168.1.1 and you have the below routes in the routing table.

192.0.0.0/8 via FastEthernet0/0
192.168.0.0/16 via FastEthernet0/1
192.168.1.0/24 via FastEthernet0/2

Which route is it going to take? Look at the prefix length. The first is a /8, the second /16 and the third /24. The /24 is longest and most specific match. The most specific is always used.

Beany

thanks for this but in the following example:

sending a packet to the host 172.16.4.3 with the routes in routing table:

172.16.1.1/32 via serial 0/1/1
172.16.1.0/24 via serial 0/1/0
172.16.0.0/22 via serial 0/1/1
172.16.0.0/16 via serial 0/1/0

the route it would take would be 172.16.0.0/16.

why is this? why does it not take the route 172.16.0.0/22 which has a higher prefix of /22?

networker050184

Because 172.16.4.3 does not fall into the 172.16.0.0/22 range.

172.16.0.0/22 = 172.16.0.0-172.16.3.255

Beany

ah cool... thanks

lantech

Remember that just because a question doesn't specifically ask you to subnet doesn't mean you won't have to subnet.

The same goes for on the job as well. You might have to do subnetting to help you isolate where the problem with the network is.

goldenlight

""sending a packet to the host 172.16.4.3 with the routes in routing table:

172.16.1.1/32 via serial 0/1/1
172.16.1.0/24 via serial 0/1/0
172.16.0.0/22 via serial 0/1/1
172.16.0.0/16 via serial 0/1/0

the route it would take would be 172.16.0.0/16.

why is this? why does it not take the route 172.16.0.0/22 which has a higher prefix of /22? ""


NICe Subnetting Question. Adding to study.

Beany

lantech wrote: »

Remember that just because a question doesn't specifically ask you to subnet doesn't mean you won't have to subnet.

The same goes for on the job as well. You might have to do subnetting to help you isolate where the problem with the network is.

I'll bear that in mind for future, thank you

Beany

guys,

im going braindead here,

can someone subnet 172.16.0.0/16 for me please? want to completely understand this topic before i move on.

thanks

networker050184

172.16.0.0/16 is an easy one. You have sixteen network bits so its 172.16.0.0-172.16.255.255.

It sounds like you are having issues with subnetting, not longest prefix match.

Beany

networker050184 wrote: »

172.16.0.0/16 is an easy one. You have sixteen network bits so its 172.16.0.0-172.16.255.255.

It sounds like you are having issues with subnetting, not longest prefix match.

i thought i was confident with subnetting but probably not looking at it for a while has made me forgetful. Was struggling to subnet ip addresses with /8 /16 /24 masks. Was getting confused with which octet needed to be increased when creating subnets from this IP.

ToLuTion

networker050184 wrote: »

Because 172.16.4.3 does not fall into the 172.16.0.0/22 range.

172.16.0.0/22 = 172.16.0.0-172.16.3.255

thanks for this but in the following example:
Adding a little twist to it. what if
sending a packet to the 172.16.1.1 address with the routes in routing table:

172.16.1.1/32 via serial 0/1/1
172.16.1.0/24 via serial 0/1/0
172.16.0.0/22 via serial 0/1/1
172.16.0.0/16 via serial 0/1/0

the route it would take would be?

why is this?
why does it not take the route ?

if longer prefix length is better then 172.16.1.1/32 seems write
but 172.16.1.0/24 seems correct also it will have the address 1720.16.1.1 in its network...
kindly explain why and how your answer is...

CCIE Wanna Be

If you need help with subnetting start here, it helped me. I know it's an older thread, but still relevant.

http://www.techexams.net/forums/ccna-ccent/38772-subnetting-made-easy.html

Heero

ToLuTion wrote: »

thanks for this but in the following example:
Adding a little twist to it. what if
sending a packet to the 172.16.1.1 address with the routes in routing table:

172.16.1.1/32 via serial 0/1/1
172.16.1.0/24 via serial 0/1/0
172.16.0.0/22 via serial 0/1/1
172.16.0.0/16 via serial 0/1/0

the route it would take would be?

why is this?
why does it not take the route ?

if longer prefix length is better then 172.16.1.1/32 seems write
but 172.16.1.0/24 seems correct also it will have the address 1720.16.1.1 in its network...
kindly explain why and how your answer is...

172.16.1.1 matches all four of those routes. Therefore, the router determines which to use via the longest prefix match rule. The /32 is the matching route with the longest prefix so it chooses that.