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IP Addressing


This section looks at IP addressing, subnet masking, Private and Special addresses. Examples are provided to illustrate the methodology when setting up an IP network addressing scheme. We also look at Wildcard masks and Directed Broadcasts.

IP Address Classes

Unique IP (Internet Protocol) addresses are assigned to each physical connection of a device to a network, therefore if a device (host) has more than one connection to a network or networks, then it will have more than one IP address.

An IP address is represented as four decimal integers, with each integer corresponding to one byte this means an IP address is 32 bits long as per the following example:-
162.            146.            93.             14              dotted decimal
10100010.       10010010.       01011101.       00001110        binary
IP addresses are divided into two parts, a Network ID and a Host ID each of which can be of varying bit lengths but always making 32 bits altogether.

Hint:- Use the Windows calculator to convert binary to decimal and vice versa.

There are five primary classes of IP addresses and it is the high order 3 bits of the address which identify the class as shown below:-
                        First Octet         Example Network         Host
Class A 0xxxxxxx        1-127                  1
Class B 10xxxxxx        128-191               1
Class C 110xxxxx        192-223                  1
Class D 1110xxxx        224-239                 1
Class E 11110000        240-254                  1
Class A addresses contain 7 bits in the network portion giving 27 - 2 = 126 possible networks since all 1's and all 0's are not allowed. Consequently 24 bits remain for the host portion allowing a total of 224 - 2 = 16,777,214 hosts. is reserved for loopback address purposes where just is used normally. The address is used as broadcast addresses and as a default route address, meaning any network. The address is sometimes used by hosts that have yet to receive an IP address e.g. a DHCP Client awaiting an address from the DHCP server.

Class B addresses contain 14 bits in the network portion allowing 214 - 2 = 16,382 possible networks, and 16 bits for the host portion allowing a possible total number of 216 - 2 = 65,534 hosts.

Class C addresses contain 21 bits for the network portion giving a possible total of 221 - 2 = 2,097,150 networks, and 8 bits for the host portion giving a possible 28 - 2 = 254 hosts.

Class D addresses are used for multicasting and Class E addresses are used in research.

Historically, a company may have been allocated just one Class A, B or C IP address by the Network Information Centre (NIC). Currently, all Class A addresses have been allocated and most if not all of the Class B addresses have gone. If a company have a number of networks to manage then the network administrator may wish to subnet his network, that is create subnet addresses within the scope of the IP address that the administrator has been given.


Subnetting Example

A customer has been given an IP address of (a Class B address) for his company. They have specified that they require 3 separate networks with the maximum possible number of host connections on each network.

The first two octets 128.100 are fixed since these are given by NIC as the Class B address, therefore we have the last two octets to play with. Let us examine the possibilities more closely:
  1. The address given
    Octet 1            Octet 2         Octet 3         Octet 4
    10000000           01100100        00000000        00000000
    128.               100.            0.              0
  1. We need to create a minimum of 3 different subnets but not at the expense of the number of host addresses available to us. The following process would seem to give us 4 permutations of subnets:
    Looking at octet 3 specifically in binary, let us just use the first 2 bits for a subnet address:
    128	64	32	16	8	4	2	1
    1	1	0	0	0	0	0	0
    The possible combinations for the first two bits are:
    11	=	192	->
    10	=	128	->
    01	=	64	->
    00	=	0	->
    However all 1s and all 0s used to be not allowed for a subnet. These subnets are called the All-ones subnet and Subnet Zero. The reason for this was that older software found it difficult to distinguish between networks and the all-zeros subnet The same was true of the all-ones subnet. RFC 950 therefore rules out '11' and '00' as useable subnets, we are therefore left with only two subnet addresses instead of the 3 we require.
  1. Let us try and use an extra bit in octet 3:
    128	64	32	16	8	4	2	1
    1	1	1	0	0	0	0	0
    The possible combinations are now:
    111	=	224	->
    110	=	192	->
    101	=	160	->
    011	=	96	->
    001	=	32	->
    010	=	64	->
    100	=	128	->
    000	=	0	->
    As before all 1s and all 0s are not permitted for subnets, therefore we are left with 6 possible subnets (23 - 2):-
  1. This leaves the rest of the bits (from power 16 downwards) in octet 3 and all the bits in octet 4 to construct the individual host addresses, the permutations amount to many thousands of hosts which should be plenty. Below is an example of a host address in subnet
    On first inspection it would appear that address has nothing to do with the subnet, so let us look a little more closely at the final two octets of the host address:
    Octet 3 = 194                            Octet 4 = 23
    128  64   32   16   8   4   2   1        128  64   32   16   8   4   2   1
    1    1    0    0    0   0   1   0        0    0    0    1    0   1   1   1
    As we can see we are indeed part of the subnet since it is only the first three bits of octet 3 which are used for the subnet address. All the bits from power 16 and downwards are allocated to the host address, so the power 2 bit just turns octet 3 from decimal 192 to decimal 194. Confusion frequently arises in this situation where the dividing line between the network portion of the IP address and the host portion rests part way through an octet (in this case between power 32 and power 16 of octet 3). Often it is possible to make the network/host dividing line between octets so that you can easily tell which host address belongs to which subnet.

    Routers are used to minimise unnecessary traffic, and when running IP it is important to tell it which subnet an address is supposed to go. The way this is done, is at configuration by entering a 'subnet mask'.
The situation with the All-zeros and All-ones subnets nowadays is to allow them according to RFC 1878. This is because modern applications understand how to distinguish between these subnets and the main network.

Subnet masks
The subnet mask specifies the portion of the IP address that is going to be used for subnetworks (as opposed to hosts). For every bit position in the IP address that is part of the network ID or subnetwork ID, a '1' is set, and for every bit position in the IP address that is part of the host id portion, a '0' is set. The router uses the boolean AND operation with an incoming IP address to 'lose' the host portion of the IP address i.e. the bits that are '0', and match the network portion with its routing table. From this, the router can determine out of which interface to send the datagram. This means that the 'Don't care bits' are represented by binary 0s whilst the 'Do care bits' are represented by binary 1s.

For our example above, because we used the first three bits in octet 3 for our subnet addressing the subnet mask would be:
Octet 1		Octet 2		Octet 3		Octet 4
11111111	11111111	11100000	00000000
255.		255.		224.		0
What is important is that the same mask is applied throughout the physical networks that share the same subnet part of the IP address. All devices connected to the networks that compose the subnet must have the same mask.

A Broadcast Address for a subnet is when all 1s are used in the host portion of the IP address. For example, for the IP address and a mask of the subnet is and the host id is 4. The broadcast address within the subnet is when the host id portion of the address is made up of all binary 1s. In this example the host portion is the last octet and if these 8 bits are set to 1 we have a broadcast address of You can ping this, send messages to this and so on, a single line to server a multitude of end stations.

Often you will see the network mask represented as a number of bits e.g. for the above example address of with a mask of, this can also be represented as, where the 24 represents 24 bits (3 octets) set to 1.

Another Subnetting Example

Study the schematic below:


The network drawing above shows the IP address map for a WAN installation carried out for a large institution. The customer had installed 'Windows NT' servers at a number of sites and was requiring an ISDN link, star-wired out, from each of the sites from the main office server room. The IP addressing scheme had to take into account the following factors:-
  • Up to 30 more sites may be added to the WAN in the near future.
  • Each site could have up to 50 host connections.
  • The customer had already assigned IP addresses to some of the servers and site PCs on the local LANs.
The IP address given to this company was (which is a Class B address), and the decision was made to use the whole of octet 3 for the subnet addresses leaving octet 4 for the host addresses. This made assigning IP addresses more easy to carry out and gave a maximum of 254 hosts per subnet and there could be a maximum of 254 subnets, thus satisfying the customer's requirements. The subnet mask for each subnet (Whether LAN or WAN) was consequently, it is important to design the addressing scheme such that the subnet mask is common to all LAN's/WAN's throughout the network unless a routing protocol such as OSPF is to be used. OSPF allows variable subnet masking.

Whilst studying the schematic you will note that the WAN links are to and the router ISDN interfaces are .20 at the main office end and .10 at the remote office end. Also you will note that the server IP addresses are all .5 and the ethernet hubs are all .8 while the router ethernet interfaces are all .6. Organising addressing like this can make life much easier especially when you are hopping from site to site.

RFC 950 and RFC 1812 describes IP subnetting whereas RFC 1009 defines Variable Length Subnet Masking.

Quick tricks to find subnets and broadcast addresses

If you have a subnet mask, then it is possible to quickly list out the possible subnets and broadcast addresses.

The number by which subnets increment for a given mask is calculated by subtracting the last numbered octet in decimal from 256. For example, given the subnet, the last numbered octet is 248, therefore 256 - 248 = 8, so subnets jump up in 8s i.e.,, etc.

Once you have found out by how much subnets jump, finding a broadcast address for each subnet is quickly done by subtracting 1 from this and adding this to each subnet. Using the above example, for subnet, the subnets jump in 8s, 8 - 1 = 7 and 8 + 7 = 15 so, taking it as given that the final octet will be all one's for the broadcast, the broadcast address is

Wildcard Masks

You will often come across Wildcard masks, particularly if you work with OSPF and/or Cisco routers. The use of wildcard masks is most prevalent when building Access Control Lists (ACLs) on Cisco routers. ACLs are filters and make use of wildcard masks to define the scope of the address filter. Although ACL wildcard masks are used with other protocols, we will concentrate on IP here.

Let us first take a simple example. We may want to filter a sub-network which has a Class C mask (24-bit) The ACL will require the scope of the addresses to be defined by a wildcard mask which, in this example is This means that the 'Don't care bits' are represented by binary 1s whilst the 'Do care bits' are represented by binary 0s. You will note that this is the exact opposite to subnet masks!

Taking a more complex example. Say we wish to filter out a subnet which is given by having a mask of i.e. How do we find the wildcard mask for this? Well to help us, concentrating on the 4th octet, let us first look at the binary for this network and subnet mask. Then we reverse the binary bits to get the wildcard bits and then convert back to decimal to obtain the wildcard mask for the 4th octet:

4th octet in decimal 32
4th octet in binary 0 0 1 0 0 0 0 0
4th octet mask in decimal 224
4th octet mask in binary 1 1 1 0 0 0 0 0
Now the 4th octet wildcard in binary 0 0 0 1 1 1 1 1
Now the 4th octet wildcard in decimal 31

The important bits have been highlighted in bold and this shows that the wildcard mask for the network is

The following table should help in seeing a pattern between the number of bits used for the mask in a particular octet, the subnet mask in decimal and the equivalent wildcard mask:

No. of Network Bits Set to 1 0 1 2 3 4 5 6 7 8
Subnet Mask Binary 00000000 10000000 11000000 11100000 11110000 11111000 11111100 11111110 11111111
Subnet Mask Decimal 0 128 192 224 240 248 252 254 255
Wildcard Mask Binary 11111111 01111111 00111111 00011111 00001111 00000111 00000011 00000001 00000000
Wildcard Mask 255 127 63 31 15 7 3 1 0

The binary for the wildcard mask is the exact reverse, bit for bit, of the subnet mask. You then calculate the decimal from the reversed binary bits to obtain the dotted decimal wildcard mask.

Private Addresses

One of the ways to combat the fast reduction in available IP address space was to introduce the concept of private addresses and the use of Network Address Translator (NAT) to allow many organisations to use the same address space but not have this space visible on the Internet i.e. to use address translation on the edge of the networks.

The Class A network address range to ( is designated for private use only. This address range cannot be used on the Internet as every ISP will automatically drop the address. This address is becoming very popular as its use in conjunction with Network Address Translation (NAT) has meant that large corporations can make use of the Class A address space available within for their own private use internally and just use NAT for those relatively few addresses that do need to operate on the Internet. This is one reason why the immediate need for IP version 6 has been diminished.

There is also the private address range to ( which is the CIDR block of 16 x Class B addresses,, .... ,

The network address range to ( is also for private use and is a CIDR block of 256 x Class C addresses,, .... ,

Examine RFC 1918 for more information on address allocation for private networks.

Other Special addresses

The address range is currently considered throughout the Internet as for special use. Note that this is different from the host address which means 'default'. You can have legitimate addresses in the range, e.g.

The address range is called the Test Net and is reserved for use in testing examples and documentation.

The address range is used for auto-configuration of IP addresses if a DHCP server should fail and there is no backup for the DHCP Clients. This is described in RFC 2563 Stateless Auto-configuration.

Directed Broadcasts

The RFC 1812 overviews the requirements of routers to run IPv4. One of the requirements is that routers MUST, by default accept Directed Broadcasts (although it is allowable to have a switch that turns this off). A directed broadcast is one where the IP broadcast has been sent to a destination prefix (a net or subnet). A directed broadcast destined for the network would be, for example.

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