Subnet Example #1
Given
An organization has been assigned the network number 193.1.1.0/24 and it needs to
define six subnets. The largest subnet is required to support 25 hosts.
Defining the Subnet Mask / Extended-Prefix Length
The first step is to determine the number of bits required to define the six subnets.
Since a network address can only be subnetted along binary boundaries, subnets
must be created in blocks of powers of two [ 2 (2 1 ), 4 (2 2 ), 8 (2 3 ), 16
(2 4 ), etc. ]. Thus, it is impossible to define an IP address block such that
it contains exactly six subnets. For this example, the network administrator must
define a block of 8 (2 3 ) and have two unused subnets that can be reserved for
future growth.
Since 8 = 2 3 , three bits are required to enumerate the eight subnets in the block. In this example, the organization is subnetting a /24 so it will need three more bits, or a /27, as the extended-network-prefix. A 27-bit extended-network-prefix can be expressed in dotted-decimal notation as 255.255.255.224. This is illustrated in Figure 11.
Figure 11: Example #1 - Defining the Subnet Mask/Extended-Prefix Length
A 27-bit extended-network-prefix leaves 5 bits to define host addresses on each subnet. This means that each subnetwork with a 27-bit prefix represents a contiguous block of 2 5 (32) individual IP addresses. However, since the all-0s and all-1s host addresses cannot be allocated, there are 30 (2 5 -2) assignable host addresses on each subnet.
Defining Each of the Subnet Numbers The eight subnets will be numbered 0 through 7. Throughout the remainder of this paper, the XXX2 notation indicates the binary representation of the number. The 3-bit binary representation of the decimal values 0 through 7 are: 0 (0002 ), 1 (0012 ), 2 (0102 ), 3 (0112 ), 4 (1002 ), 5 (1012 ), 6 (1102 ), and 7 (1112 ).
In general, to define Subnet #n, the network administrator places the binary representation of n into the bits of the subnet-number field. For example, to define Subnet #6, the network administrator simply places the binary representation of 6 (1102 ) into the 3-bits of the subnet-number field.
The eight subnet numbers for this example are given below. The italicized portion
of each address identifies the extended-network-prefix, while the bold digits
identify the 3- bits representing the subnet-number field:
Base Net : 11000001.00000001.00000001 .00000000 = 193.1.1.0/24
Subnet #0: 11000001.00000001.00000001.000 00000 = 193.1.1.0/27
Subnet #1: 11000001.00000001.00000001.001 00000 = 193.1.1.32/27
Subnet #2: 11000001.00000001.00000001.010 00000 = 193.1.1.64/27
Subnet #3: 11000001.00000001.00000001.011 00000 = 193.1.1.96/27
Subnet #4: 11000001.00000001.00000001.100 00000 = 193.1.1.128/27
Subnet #5: 11000001.00000001.00000001.101 00000 = 193.1.1.160/27
Subnet #6: 11000001.00000001.00000001.110 00000 = 193.1.1.192/27
Subnet #7: 11000001.00000001.00000001.111 00000 = 193.1.1.224/27
An easy way to check if the subnets are correct is to ensure that they are all multiples of the Subnet #1 address. In this case, all subnets are multiples of 32: 0, 32, 64, 96, ...
The All-0s Subnet and The All-1s Subnet
When subnetting was first defined in RFC 950, it prohibited the use of the all-0s
and the all-1s subnet. The reason for this restriction was to eliminate situations
that could potentially confuse a classful router. Note that today a router can
be both classless and classful at the same time - it could be running RIP-1 (a
classful protocol) and BGP-4 (a classless protocol) at the same time.
With respect to the all-0s subnet, a router requires that each routing table update include the route/<prefix-length> pair to differentiate between a route to the all-0s subnet and a route to the entire network. For example, when using RIP-1 which does not supply a mask or prefix-length with each route, the routing advertisements for subnet 193.1.1.0/27 and for network 193.1.1.0/24 are identical - 193.1.1.0. Without somehow knowing the prefix-length or mask, a router cannot tell the difference between a route to the all-0s subnet and the route to the entire network! This is illustrated in Figure 12.
Figure 12: Differentiating Between a Route to the All-0s Subnet and the Entire Network
Regarding the all-1s subnet, a router requires that each routing table entry include the prefix-length so that it can determine if a broadcast (directed or all-subnets) should be sent only to the all-1s subnet or to the entire network.For example, when the routing table does not contain a mask or prefix-length for each route, confusion can occur because the same broadcast address (193.1.1.255) is used for both for the entire network 193.1.1.0/24 and the all-1s subnet 193.1.1.224/27. This is illustrated in Figure 13.
Figure 13: Identifying a Broadcast to the All-1s Subnet and the Entire Network
Note that by default, NETBuilder software permits the forwarding of traffic to a directed broadcast address but does not forward traffic to the all-subnets broadcast address. The network administrator can modify this behavior via the -IP CONTrol parameter switches FwdSubnetBcast | NoFwdSubnetBcast and FwdAllSubnetBcast | NoFwdAllSubnetBcast.
With the development of routing protocols that supply the mask or prefix-length with each route, the address space defined by the all-0s and all-1s subnets is once again usable despite the cautions in RFC 950. As a result, vendors have begun to accommodate user demand and permit the configuration of the all-0s and all-1s subnets on router interfaces. There are three factors that determine when these subnets can be used with NETBuilder software.
NETBuilder software has taken a phased approach in its support for the deployment of the all-0s and all-1s subnets.Table 2 shows the NETBuilder software version number that first implemented the various features that support the deployment of these subnets.
Table 2: NETBuilder Software Features Supporting the All-0s and All-1s Subnets
Finally, the other routers in the organization's network need to be able to correctly interpret, learn, and forward traffic to other subnetworks with all-0s and all-1s in their subnet number field.
Defining Host Addresses for Each Subnet
According to Internet practices, the host-number field of an IP address cannot contain
all 0-bits or all 1-bits. The all-0s host-number identifies the base network
(or subnetwork) number, while the all-1s host-number represents the broadcast
address for the network (or subnetwork).
In our current example, there are 5 bits in the host-number field of each subnet address. This means that each subnet represents a block of 30 host addresses (2 5 -2 = 30, note that the 2 is subtracted because the all-0s and the all-1s host addresses cannot be used). The hosts on each subnet are numbered 1 through 30.
In general, to define the address assigned to Host #n of a particular subnet, the network administrator places the binary representation of n into the subnet's host-number field. For example, to define the address assigned to Host #15 on Subnet #2, the network administrator simply places the binary representation of 15 (011112 ) into the 5-bits of Subnet #2's host-number field.
The valid host addresses for Subnet #2 in our example are given below. The italicized
portion of each address identifies the extended-network-prefix, while the bold
digits identify the 5-bit host-number field:
Subnet #2: 11000001.00000001.00000001.010 00000 = 193.1.1.64/27
Host #1: 11000001.00000001.00000001.010 00001 = 193.1.1.65/27
Host #2: 11000001.00000001.00000001.010 00010 = 193.1.1.66/27
Host #3: 11000001.00000001.00000001.010 00011 = 193.1.1.67/27
Host #4: 11000001.00000001.00000001.010 00100 = 193.1.1.68/27
Host #5: 11000001.00000001.00000001.010 00101 = 193.1.1.69/27
.
.
Host #15: 11000001.00000001.00000001.010 01111 = 193.1.1.79/27
Host #16: 11000001.00000001.00000001.010 10000 = 193.1.1.80/27
.
.
Host #27: 11000001.00000001.00000001.010 11011 = 193.1.1.91/27
Host #28: 11000001.00000001.00000001.010 11100 = 193.1.1.92/27
Host #29: 11000001.00000001.00000001.010 11101 = 193.1.1.93/27
Host #30: 11000001.00000001.00000001.010 11110 = 193.1.1.94/27
The valid host addresses for Subnet #6 are given below. The italicized portion of
each address identifies the extended-network-prefix, while the bold digits identify
the 5-bit host-number field:
Subnet #6: 11000001.00000001.00000001.110 00000 = 193.1.1.192/27
Host #1 : 11000001.00000001.00000001.110 00001 = 193.1.1.193/27
Host #2 : 11000001.00000001.00000001.110 00010 = 193.1.1.194/27
Host #3 : 11000001.00000001.00000001.110 00011 = 193.1.1.195/27
Host #4 : 11000001.00000001.00000001.110 00100 = 193.1.1.196/27
Host #5 : 11000001.00000001.00000001.110 00101 = 193.1.1.197/27
.
.
Host #15 : 11000001.00000001.00000001.110 01111 = 193.1.1.207/27
Host #16 : 11000001.00000001.00000001.110 10000 = 193.1.1.208/27
.
.
Host #27 : 11000001.00000001.00000001.110 11011 = 193.1.1.219/27
Host #28 : 11000001.00000001.00000001.110 11100 = 193.1.1.220/27
Host #29 : 11000001.00000001.00000001.110 11101 = 193.1.1.221/27
Host #30 : 11000001.00000001.00000001.110 11110 = 193.1.1.222/27
Defining the Broadcast Address for Each Subnet
The broadcast address for Subnet #2 is the all 1's host address or:
11000001.00000001.00000001.01011111 = 193.1.1.95
Note that the broadcast address for Subnet #2 is exactly one less than the base address for Subnet #3 (193.1.1.96). This is always the case - the broadcast address for Subnet #n is one less than the base address for Subnet #(n+1).
The broadcast address for Subnet #6 is simply the all 1's host address or:
11000001.00000001.00000001.11011111 = 193.1.1.223
Again, the broadcast address for Subnet #6 is exactly one less than the base address
for Subnet #7 (193.1.1.224).