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I'm learning (or trying to) from the Windows Networking Essentials book. I'm struggling with the concept of subnetting. I've read the pages again, googled it, been for a walk and I still don't get it!

The book gives an example of a network with two subnets, like so:

Subnet 1

Network id:
Subnet mask:

Subnet 2

Network id:
Subnet mask:

I don't understand why we can't just do this:

Subnet a

Network id:
Subnet mask:

Subnet b

Network id:
Subnet mask:

I don't understand why one would borrow bits in the subnet mask as in 1 & 2, why wouldn't you just use a different network id like a & b?

It may be that I'm totally misunderstanding this, and if you have the Windows Networking Essentials book it's pp.100-108 :)

marked as duplicate by Ron Maupin, Teun Vink Jan 6 '16 at 19:41

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It's all about having more flexibility on how you can allocate your addresses.

Subnet 1 provides less host addresses than Subnet a

Thus Subnet 1 is an option for you to save addresses (as compared to Subnet a) for use in OTHER subnets.

Subnet 2 provides more host addresses than Subnet b

Thus Subnet 2 is an option for you build a larger LAN (as compared to Subnet b).

  • 1
    So all of the subnets I've listed in the question are valid, it's just a question of which is most appropriate for that particular network? – frankle Jan 6 '16 at 14:20
  • Yes. Subnetting is short term for "breaking an address block into smaller blocks". – Everton Jan 6 '16 at 15:16

One of the important factors to think about when subnetting is the size of your network (the number of host addresses you need). Different subnet masks, i.e. borrowing bits from the host part of the address, allow you to break a large network into a bunch of smaller networks. For example, in Subnet 1 with subnet mask (, we have the following situation:

  • We see that bits for the subnet mask have been borrowed from the 4th octet|000000
  • 2^(number of bits we've borrowed) gives us the number of different subnets we can make - in this case 4
  • Each subnet will have 2^(remaining bits) = 2^6 - 2 = 62 host addresses (we take out the broadcast and the network address)

Subnetting is a good choice when you have been assigned one large address block and need to have multiple small networks (for example, different networks for each floor in your company). This helps to reduce network congestion and is easier and more flexible to administrate.

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