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Chapter 5
Subnetting
And
Supernetting
Hyung Min Lee
Subnetting and Supernetting
About subnetting & Supernetting
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In subnetting, a network is divided into several smaller subnetwork with each
subnetwork(or subnet) having its own subnetwork address.
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In supernetting, an organization can combine several class C addresses to create a
larger range of addresses(supernetwork).
5.1 SUBNETTING
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Class A, B, C in IP addressing are designed with two levels of hierarchy.(netis and
hostid)
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The organization has two-level hierarchical addressing, but it cannot have more than one
physical network./ The host cannot be organized into groups, and all of the hosts are at
the same level./ The organization has one network with many hosts.
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One solution to this problem is subnetting, the further division of a network into smaller
networks called subnetworks.
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Figure 5-1 A network with two levels of hierarchy (not subnetted)
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Figure 5-2 A network with three levels of hierarchy (subnetted)
The router R1 uses the first two octet(141.14)as the netid, the third
octet(2) as the subnetid, and the fourth octet(21) as the hostid.
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Subnetting and Supernetting
Three Levels of Hierarchy
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Adding subnetworks creates an intermediate level of hierarchy in the IP
addressing system.
– Netid:
It defines the site.
– Subnetid : It defines the physical subnetwork
– Hostid:
It defines the connection of the host to the network.
Figure 5-3 Addresses in a network with and without subnetting
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Figure 5-4 Hierarchy concept in a telephone number
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Subnetting and Supernetting
5.2 MASKING
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Masking is a process that extracts the address of the physical network from an
IP address.
Masking can be done whether we have subnetting or not.
– Not subnetted the network: Masking extracts the network address an IP
address.
– Subnetted the network: Masking extracts the subnetwork address an IP
address.
In masking, we perform a mathematical operation on a 32-bit IP address at the
bit level using another 32-bit number called the mask.
To get the network or subnetwork address, we must apply the bit-wise-and
operation on the IP address an the mask.
The part of the mask containing 1s defines the netid(network portion) or
combination of netid and subnetid(subnetwork portion), The part of the mask
containing 0s defines the hostid.
Figure 5-5 Masking
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Figure 5-6 Applying bit-wise-and operation to achieve masking
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Subnetting and Supernetting
Special Addresses in Subnetting
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A subnetid of all 1s or all 0s is not assigned to any host.
The address with the hostid of all 1s is reserved for broadcasting to all hosts in a
specific subnet.
The address with the hostid of all 0s is also reserved to define the subnetwork itself.
Figure 5-7 Special addresses in subnetting
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Subnetting and Supernetting
5.3 EXAMPLES OF SUBNETTING
( Example 1 / Class A)
An organization with a class A address at least 1,000 subnetworks. Find the subnet mask and
configuration of each subnetwork.
(Solution)
1.
There is a need for at least 1,000 subnetworks.(we need at least 1,002 subnetworks to
allow for the all-1s and all-0s subnetids)
2.
This means that the minimum number of bits to be allocated for subnetting should be
10, (29 < 1,002 < 210).
3.
14 bits are left to define the hostids. Subnet mask (11000000 = 192).
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Figure 5-8 Masks in example 1
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Figure 5-9 Range of addresses in example 1
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Figure 5-10 Subnetworks in example 1
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Subnetting and Supernetting
( Example 2 / Class B)
An organization with a class B address needs at least 12 subnetworks. Find the subnet mask
and configuration of each subnetwork.
(Solution)
1.
There is a need for at least 14 subnetworks.(we need at least 14 subetworks to allow
for the all-1s and all-0s subnetids)
2.
This means that the minimum number of bits to be allocated for subnetting should be
4, (23 < 14 < 24).
3.
12 bits are left to define the hostids. Subnet mask (11110000 = 240).
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Figure 5-11 Masks in example 2
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Figure 5-12 Range of addresses in example 2
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Figure 5-13 Subnetworks in example 2
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Subnetting and Supernetting
( Example 3 / Class C)
An organization with a class C address needs at least 5 subnetworks. Find the subnet mask
and configuration of each subnetwork.
(Solution)
1.
There is a need for at least 7 subnetworks.(we need at least 14 subetworks to allow
for the all-1s and all-0s subnetids)
2.
This means that the minimum number of bits to be allocated for subnetting should be
3, (22 < 7 < 23).
3.
5 bits are left to define the hostids. Subnet mask (11100000 = 224).
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Figure 5-14
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Masks in example 3
Figure 5-15 Range of addresses in example 3
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Figure 5-16 Subnetworks in example 3
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Subnet Short Cut Table
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# of Bits
1
2
3
4
5
6
7
8
Incrementing
Value
128
64
32
16
8
4
2
1
Subnet Mask
128
192
224
240
248
252
254
255
# of
Networks(2n-2)
0
2
6
14
30
62
126
254
Subnetting and Supernetting
5.4 VARIABLE-LENGTH SUBNETTING
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The Internet allows a site to use variable-length subnetting.
( Example)
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An organization with a class C address and needs to have 5 subnets with the
following number of hosts: 60, 60, 60, 30, 30.
(Solution)
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The site cannot use a subnet mask with only 2 bits in the subnet section
because this allows only 4 subnetworks each 62 hosts(256/4 – 2 = 62),
(22<5<23)
Nor can the site use a subnet mask with 3 bits in the subnet section because
this allows 8 subnetworks each with 30 hosts(256/8 – 2 – 30).
(Variable length subnetting ) / The router uses 2 different masks, one applied
after the other.
It first uses the masks with 26 1s(11111111 11111111 11111111 11000000 or
255.255.255.192) to divide the network into 4 subnets.
Then it applies the mask with 27 1s (11111111 11111111 11111111 11100000
or 255.255.255.224) to one of the subnets to divide it into two smaller subnets.
Figure 5-17 Variable-length subnetting
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Subnetting and Supernetting
5.5 SUPPERNETTING
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4 class C addresses combine to make one supernetwork. Figure 5-18
Suppernet Mask
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A supernet mask is the reverse of the subnet mask.
In a supernet mask, we change some 1s in the netid section to 0s.
Be aware that the position of 1s in the supernet mask defines the lowest address.
( Example)
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With the supernet mask of 255.255.252.0 we can have 4 class C address combined
into one supernetwork.
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If we choose the first address to be X.Y.32.0, the other three addresses are X.Y.33.0,
X.Y.34.0, and X.Y.35.0.
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Whenever the router receives a packet, it applies the supernet mask to the destination
address and compares the result to the lowest address. If the result and the lowest
address are the same, the packet belongs to the supernet.
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Figure 5-18 Supernetwork
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Figure 5-19 Supernet mask
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Figure 5-20 Two ways of defining a supernet
210 = 1024
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Figure 5-21 Example of supernetting
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Subnetting and Supernetting
Classless Interdomain Routing(CIDR)
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Supernetting means assigning a set of class C addresses to an organization that
needs more than 254 host addresses.
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However , when these class C addresses are entered into the routing table, each
occupies one entry in the routing table./ 256 entries in the routing table.
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The classless interdomain routing(CIDR) technique is devised to reduce the
number of routing table entries.
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In this technique, instead of entering each single class C address with its
corresponding default mask(255.255.255.0), the router can use the supernet mask
and the lowest network address in the group.
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Figure 5-22 CIDR
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