Overlapping IP ranges and routing

ICND 2 tells me that if 2 routes match a destination IP of 172.16.4.3, the route with the longest prefix will be used to send the packet.

SHOW IP ROUTE RIP

172.16.0.0/16 is variably subnetted, 5 subnets, 4 masks

172.16.1.1/32

172.16.1.0/24

172.16.0.0/22

172.16.0.0/16

0.0.0.0/0

SHOW IP ROUTE 172.16.4.3

Routing entry for 172.16.0.0/16

The book then says the route to be taken is 172.16.0.0/16.

Could this be a mis-print?

I would think the chosen route would be 172.16.0.0/22 since it has a higher prefix.

Find more posts tagged with

Free for TechExams community: Cybersecurity salary guide

Compare cert salaries and plan your next career move

Button

Comments

2URGSE

Nevermind, I figured it out.

Longest prefix is 16 (255.255.0.0) vs. 255.255.252.0

wbosher

A prefix of 22 would include the range of 172.16.0.0 - 172.16.3.255.

A prefix of 16 would be 172.16.0.0 - 172.16.255.255

The book is correct.

AlanJames

most specific route wins

HeaVenS

Book may be referring to RIP v1 or IP Classful routing. 22 is the preferred more specific route.

mattrgee

HeaVenS wrote: »

Book may be referring to RIP v1 or IP Classful routing. 22 is the preferred more specific route.

The .22 route doesn't include the IP 172.16.4.3.

PhildoBaggins

I always think about it in binary. The more specific it in in binary is the correct route to take.

chmorin

wbosher wrote: »

A prefix of 22 would include the range of 172.16.0.0 - 172.16.3.255.

A prefix of 16 would be 172.16.0.0 - 172.16.255.255

The book is correct.

Yup. The most specific route... that works! That last bit got me a few times too =p

notun1

2URGSE wrote: »

ICND 2 tells me that if 2 routes match a destination IP of 172.16.4.3, the route with the longest prefix will be used to send the packet.

SHOW IP ROUTE RIP

172.16.0.0/16 is variably subnetted, 5 subnets, 4 masks

172.16.1.1/32

172.16.1.0/24

172.16.0.0/22

172.16.0.0/16

0.0.0.0/0

SHOW IP ROUTE 172.16.4.3

Routing entry for 172.16.0.0/16

The book then says the route to be taken is 172.16.0.0/16.

Could this be a mis-print?

I would think the chosen route would be 172.16.0.0/22 since it has a higher prefix.

:

Only matching found in 172.16.0.0/16 for 172.16.4.3 address. the address range for 172.16.0.0/22 is 172.16.0.0 - 172.16.3.255.

If you have a address like 172.16.1.3 then 172.16.0.0/22 and 172.16.0.0/16 need to consider. in this case highest prefix /22 win(More specific).