Subnetting

[sanjo]

I am configuring a network based on the Hands-on Lab in Chapter 7 of the Lammle book. The network is class B with a block of 10 ip addresses assigned to each router interface. In order to configure this network, I need a subnet mask. The subnet mask that I came up with is 255.255.224.0 (CIDR 19). This means that each router interface has a block of 16 ip addresses instead of 10, which means that the ip addresses seem to be overlapping. Can someone please make sense of my ramblings and tell me how I can keep the blocks of 10 ip addresses and what subnet mask I should use?:confused:

[lammle]

you are saying you need 10 hosts per network, right? I just want to make sure I am reading this question correctly.
This would be a block size of 16, or a 240 mask. You can either use a /20 or a /28, each one would give you 14 hosts, but the /28 only provides 16 subnets and the /20 provides over 4000 subnets (2^12)
Cheers!
Todd

[sanjo]

you are saying you need 10 hosts per network, right? I just want to make sure I am reading this question correctly.
This would be a block size of 16, or a 240 mask. You can either use a /20 or a /28, each one would give you 14 hosts, but the /28 only provides 16 subnets and the /20 provides over 4000 subnets (2^12)
Cheers!
Todd

Okay, but if the smallest block that I can use is 16, then why do you have blocks of 10 hosts per subnet on the Internetwork in the Chapter 7 Hands-on Lab (see CCNA, 6th ed, pg 478)?

[Bravo020]

I believe that the example shown in the hands-on lab is using a /24 mask (255.255.255.0) on that basis the block size is 256 based on the 4th octet.

Try not to be confused by the numbering of the private class B network, the spreading of the network address by 10 plays no part in the block size.

The neworks and their valid host ranges and broadcast addresses are:

Network/CIDR Mask Valid Hosts Broadcast
172.16.10.0/24 255.255.255.0 172.16.10.1-254 172.16.10.255
172.16.20.0/24 255.255.255.0 172.16.20.1-254 172.16.20.255
172.16.30.0/24 255.255.255.0 172.16.30.1-254 172.16.30.255
172.16.40.0/24 255.255.255.0 172.16.40.1-254 172.16.40.255
172.16.50.0/24 255.255.255.0 172.16.50.1-254 172.16.50.255
This still means that other networks are still available but have not been used, 172.16.11.0, 172.16.12.0, 172.16.13.0 etc.

If you wanted to subnet the networks into even smaller block sizes than 256, for example 16 block size, then a mask of 255.255.255.240 could be used, and then they could be divided as follows:

Network/CIDR Mask Valid Hosts Broadcast
172.16.10.0/28 255.255.255.240 172.16.10.1-14 172.16.10.15
172.16.10.16/28 255.255.255.240 172.16.10.17-30 172.16.10.31
172.16.10.32/28 255.255.255.240 172.16.10.33-46 172.16.10.47I hope that this helps bring some light to the subject.

Regards.