Subnetting and Security+

Posted on by Darril Gibson

I recently received the following question on subnetting and realized I couldn’t adequately answer it without graphics so I’m including the answer here.

“I’m in the process of studying for the Security+ exam but believe I found a glaring error in the book. I’ve been over it several different ways, but the subnetting explanation on page 143 (and the followup on page 144) doesn’t make sense.

“For “subnet 1”, a mask of .192 works, but to expand the range of IP addresses for “subnet 2”, you’d need to devote another bit to the host address, thus reducing the mask to .128.  Technically this range also includes the addresses from “subnet 1”, but could be worked around if they’re supplied by a different DHCP server (or coded statically). It appears “subnet 3” would work with a .64 mask, but as this would be a non-contiguous mask, I know it isn’t recommended. Likewise “subnet 4” would need the default mask of 255.255.255.0 to accommodate those 8-bit addresses. Like “subnet 2″, subnets 3 and 4 will include addresses from the other subnets unless a solution exists on the administrative side.”

Here’s my answer.

Subnetting Problem with One Network

It’s often useful to look at this from a problem/resolution perspective.

You start with one network: 192.168.1.0/24. This includes all the usable addresses from 192.168.1.1 to 192.168.1.254 with a subnet mask of 255.255.255.0. It might look like the following graphic:

Subnetting - starting with one network

Here’s the problem. All the computers on this one network are competing for the same bandwidth, effectively slowing down the entire network.

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Subnetting Solution with Four Networks

The book mentions four separate groups of users:

  1. Some of the users may be running applications that stream audio and video across the network.
  2. A second group of users may regularly upload and download data via the Internet.
  3. A third group may upload and download files back and forth to servers on the network
  4. A fourth group could be users with just occasional access to the network

With subnetting, you can divide the one big network into four smaller networks. The following graphic shows the logical view of a subnetted network. You’re essentially creating a subnet for each of the four groups.

Subnetting into Four Networks

Subnetting Solution IP Addresses

Remember, the original network (192.168.1.0/24) included this range of address:

  • 192.168.1.1 to 192.168.1.254
  • Subnet mask of 255.255.255.0
    • If you look at the subnet as bits, it looks like this:
    • 1111 1111 . 1111 1111 . 1111 1111 . 0000 0000
    •     255      .       255    .      255     .          0

To divide this into four subnets, you need to use two bits from the last octet of the subnet mask.  Here is what the subnet mask now looks like:

  • 1111 1111 . 1111 1111 . 1111 1111 . 1100 0000
  •         255   .       255     .       255    .        192
  • You can also use CIDR notation to represent this as /26 because you are using 26 1 bits in the subnet mask.

Now you can create four subnets instead of one network. As an overview, the four subnets are:

  • 1100 0000 . 1010 1000 . 0000 0001 . 0000 0000
  •      192         .       168       .           1        .         0
  • 1100 0000 . 1010 1000 . 0000 0001 . 0100 0000
  •      192         .       168        .           1         .        64
  • 1100 0000 . 1010 1000 . 0000 0001 . 1000 0000
  •      192         .       168        .          1        .        128
  • 1100 0000 . 1010 1000 . 0000 0001 . 1100 0000
  •      192         .       168        .           1         .       192
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Subnetting Solution Network 1

Here’s the first network in both decimal and bits:

  • 1100 0000 . 1010 1000 . 0000 0001 . 0000 0000
  •      192         .       168       .          1        .           0

While it looks similar to the original subnet, the different subnet mask limits how many IP addresses you can use to only 62 IP addresses. Here’s the range:

  • 1100 0000 . 1010 1000 . 0000 0001 . 0000 0001
  •      192         .      168        .          1         .           1           (This is the first IP address in the subnet)
  • 1100 0000 . 1010 1000 . 0000 0001 . 0011 1110
  •      192         .      168        .          1         .         62         (This is the last IP address in the subnet)

Notice that you can’t use 192.168.1.0 as an IP address in this subnet because it identifies the subnet (192.168.1.0).

  • 1100 0000 . 1010 1000 . 0000 0001 . 0000 0000
  •      192        .       168        .          1         .           0         (Network identifier)

Also, you can’t use 192.168.1.63 as an IP address in this subnet, because it is the broadcast address for this subnet.

  • 1100 0000 . 1010 1000 . 0000 0001 . 0011 1111
  •      192        .      168       .          1          .         63         (Broadcast address)

Subnetting Solution for Network 2

Take a look at subnet 2.

  • 1100 0000 . 1010 1000 . 0000 0001 . 0100 0001
  •      192        .       168       .          1         .         65           (This is the first IP address in the subnet)
  • 1100 0000 . 1010 1000 . 0000 0001 . 0111 1110
  •      192        .      168       .          1         .         126         (This is the last IP address in the subnet)

Notice that you can’t use 192.168.1.64 as an IP address in this subnet because it identifies the subnet (192.168.1.64).

  • 1100 0000 . 1010 1000 . 0000 0001 . 0100 0000
  •      192        .      168       .          1         .         64         (Network identifier)

Also, you can’t use 192.168.1.127 as an IP address in this subnet, because this is the broadcast address for the subnet.

  • 1100 0000 . 1010 1000 . 0000 0001 . 0111 1111
  •      192        .      168       .          1         .       127         (Broadcast address)

Subnetting Solution for Network 3

Take a look at subnet 3.

  • 1100 0000 . 1010 1000 . 0000 0001 . 1000 0001
  •      192        .      168       .          1         .        129           (This is the first IP address in the subnet)
  • 1100 0000 . 1010 1000 . 0000 0001 . 1011 1110
  •      192        .      168       .          1         .        190         (This is the last IP address in the subnet)

Notice that you can’t use 192.168.1.128 as an IP address in this subnet because it identifies the subnet (192.168.1.128).

  • 1100 0000 . 1010 1000 . 0000 0001 . 1000 0000
  •      192        .      168       .          1         .         128         (Network identifier)

Also, you can’t use 192.168.1.127 as an IP address in this subnet, because this is the broadcast address for the subnet.

  • 1100 0000 . 1010 1000 . 0000 0001 . 1011 1111
  •      192        .      168       .          1        .       191         (Broadcast address)

Subnetting Solution for Network 4

Take a look at subnet 4.

  • 1100 0000 . 1010 1000 . 0000 0001 . 1100 0001
  •      192        .       168       .          1        .        193           (This is the first IP address in the subnet)
  • 1100 0000 . 1010 1000 . 0000 0001 . 1111 1110
  •      192        .       168       .          1        .        254         (This is the last IP address in the subnet)

Notice that you can’t use 192.168.1.64 as an IP address in this subnet because it identifies the subnet (192.168.1.64).

  • 1100 0000 . 1010 1000 . 0000 0001 . 1100 0000
  •      192        .      168       .          1        .        192         (Network identifier)

Also, you can’t use 192.168.1.127 as an IP address in this subnet, because this is the broadcast address for the subnet.

  • 1100 0000 . 1010 1000 . 0000 0001 . 1111 1111
  •      192        .      168       .          1         .       255         (Broadcast address)

 

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