Subnet challenge

Andrew Newman

Hi, I don't understand why I can't use subnet 192.168.9.96 /27 in the following example which is from the boson test.I hope you can help me understand it

The internetwork in the diagram uses only private network 192.168.9.0 and RIP2.A new network engineer's manager briefly started configuring the routers, configuring only router R3.The manager says that the engineer should not change R3's IP addresses, but continue the project, choosing subnet numbers and IP addresses for the remaining routers' interfaces.The design goals state that all the remaining LAN-based subnets should use a /27 mask.With all Wan links using a /30 mask.You should assign the smallest available subnet numbers to the LAN-subnets first and then assign the then-smallest-available subnet numbers for any remaining wan subnets.You should then choose interface IP addresses for the routers that are the smallest available IP addresses in each subnet.
The interfaces on R3 are as follows:
FA0/1 192.168.9.33 255.255.255.224
S001 192.168.9.101 255.255.255.252
s010 192.168.9.105 255.255.255.252

Sorry I can't copy and paste the network diagram but there are 3 other LAN subnets to assign subnet numbers to and 2 other routers with

mikej412

Andrew Newman wrote: »

You should assign the smallest available subnet numbers to the LAN-subnets first and then assign the then-smallest-available subnet numbers for any remaining wan subnets.

Andrew Newman wrote: »

The interfaces on R3 are as follows:
FA0/1 192.168.9.33 255.255.255.224
S001 192.168.9.101 255.255.255.252
s010 192.168.9.105 255.255.255.252

Andrew Newman wrote: »

Hi, I don't understand why I can't use subnet 192.168.9.96 /27

The 192.168.9.96/27 subnet would overlap with the existing networks configured on the Serial Ports -- so that subnet is NOT available for that 4th network. But that subnet has already been further subdivided for the R3 serial ports and the other unused subnets in that range will be available.

Your 4th network would have to use the 192.168.9.129/27 range.

And when you go to assign that next serial link -- the smallest available subnet would be 192.168.9.96/30

192.168.9.100/30 is already in use on one of R3's serial ports
192.168.9.104/30 is already on use on the other R3 serial port.

The next-available smallest subnet available for another serial interface would then be 192.168.9.108/30

And if I counted on my fingers correctly -- out of that 192.168.9.96/27 block, there's still a 192.68.9.112/28 piece that is unused. That could be used "as is" or broken up into 2 /29 networks or broken up into 4 /30 networks. Or that remaining piece could be split into 1 /29 and 2 /30 subnet (or 2 /30 and 1 /29 subnets depending on how you split and use it).