Network Masks:
A network
mask helps you know which portion of the address identifies the network and
which portion of the address identifies the node.
Class A, B, and C networks
have default masks, also known as natural masks, as shown here:
Class A:
255.0.0.0
Class B:
255.255.0.0
Class C:
255.255.255.0
An IP
address on a Class A network that has not been subnetted would have an
address/mask pair similar to: 8.20.15.1 255.0.0.0.
To see how the mask helps
you identify the network and node parts of the address, convert the address and
mask to binary numbers.
8.20.15.1 =
00001000.00010100.00001111.00000001
255.0.0.0 =
11111111.00000000.00000000.00000000
Once you
have the address and the mask represented in binary, then identifying the
network and host ID is easier. Any address bits which have corresponding mask
bits set to 1 represent the network ID.
Any address bits that have
corresponding mask bits set to 0 represent the node ID.
8.20.15.1 = 00001000.00010100.00001111.00000001
255.0.0.0 =
11111111.00000000.00000000.00000000
-----------------------------------
net id | host
id
netid = 00001000 =
8
hostid =
00010100.00001111.00000001 = 20.15.1
Understanding
Subnetting:
Subnetting
allows you to create multiple logical networks that exist within a single Class
A, B, or C network. If you do not subnet, you are only able to use one network
from your Class A, B, or C network, which is unrealistic.
Each data
link on a network must have a unique network ID, with every node on that link
being a member of the same network.
If you break a major network (Class A, B,
or C) into smaller subnetworks, it allows you to create a network of
interconnecting subnetworks. Each data link on this network would then have a
unique network/subnetworkID.
Any device, or gateway, connecting n
networks/subnetworks has n distinct IP addresses, one for each network /
subnetwork that it interconnects.
In order to
subnet a network, extend the natural mask using some of the bits from the host
ID portion of the address to create a subnetwork ID.
For example, given a Class
C network of 204.17.5.0 which has a natural mask of 255.255.255.0, you can
create subnets in this manner:
204.17.5.0 = 11001100.00010001.00000101.00000000
255.255.255.224 = 11111111.11111111.11111111.11100000
--------------------------|sub|----
By extending
the mask to be 255.255.255.224, you have taken three bits (indicated by
"sub") from the original host portion of the address and used them to
make subnets.
With these three bits, it is possible to create eight subnets.
With the remaining five host ID bits, each subnet can have up to 32 host addresses,
30 of which can actually be assigned to a device since host ids of all zeros or
all ones are not allowed (it is very important to remember this).
So, with this
in mind, these subnets have been created.
204.17.5.0
255.255.255.224 host address range 1 to 30
204.17.5.32
255.255.255.224 host address range 33 to 62
204.17.5.64
255.255.255.224 host address range 65 to 94
204.17.5.96
255.255.255.224 host address range 97 to 126
204.17.5.128
255.255.255.224 host address range 129 to 158
204.17.5.160
255.255.255.224 host address range 161 to 190
204.17.5.192 255.255.255.224
host address range 193 to 222
204.17.5.224
255.255.255.224 host address range 225 to 254
Examples:
Sample Exercise 1
Now that you have an
understanding of subnetting, put this knowledge to use. In this example, you
are given two address / mask combinations, written with the prefix/length
notation, which have been assigned to two devices. Your task is to determine if
these devices are on the same subnet or different subnets. You can do this by
using the address and mask of each device to determine to which subnet each
address belongs.
DeviceA: 172.16.17.30/20
DeviceB: 172.16.28.15/20
Determining the Subnet
for DeviceA:
172.16.17.30 -
10101100.00010000.00010001.00011110
255.255.240.0 - 11111111.11111111.11110000.00000000
-----------------| sub|------------
subnet = 10101100.00010000.00010000.00000000 = 172.16.16.0
Looking at the address
bits that have a corresponding mask bit set to one, and setting all the other
address bits to zero (this is equivalent to performing a logical
"AND" between the mask and address), shows you to which subnet this
address belongs. In this case, DeviceA belongs to subnet 172.16.16.0.
Determining the Subnet
for DeviceB:
172.16.28.15 - 10101100.00010000.00011100.00001111
255.255.240.0 - 11111111.11111111.11110000.00000000
-----------------|
sub|------------
subnet = 10101100.00010000.00010000.00000000
= 172.16.16.0
From these
determinations, DeviceA and DeviceB have addresses that are part of the same
subnet.
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