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SUBNET MASKA

An IP address is divided into two parts: network and host parts. For example, an IP class A address consists of 8 bits identifying the network and 24 bits identifying the host. This is because the default subnet mask for a class A IP address is 8 bits long. (or, written in dotted decimal notation, 255.0.0.0). What does it mean? Well, like an IP address, a subnet mask also consists of 32 bits. Computers use it to determine the network part and the host part of an address. The 1s in the subnet mask represent a network part, the 0s a host part.

Computers works only with bits. The math used to determine a network range is binary AND.

Let’s say that we have the IP address of 10.0.0.1 with the default subnet mask of 8 bits (255.0.0.0).

First, we need to convert the IP address to binary:

IP address: 10.0.0.1 = 00001010.00000000.00000000.00000001

Subnet mask 255.0.0.0 = 11111111.00000000.00000000.0000000

Computers then use the AND operation to determine the network number:

The computer can then determine the size of the network. Only IP addresses that begins with 10 will be in the same network. So, in this case, the range of addresses in this network is 10.0.0.0 – 10.255.255.255.

NOTE

A subnet mask must always be a series of 1s followed by a series of 0s.

PRIVIOUS TOPIC:-SUBNETTING EXPLAINED

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SUBNETTING EXPLAINED

Subnetting is the practice of dividing a network into two or more smaller networks. It increases routing efficiency, enhances the security of the network and reduces the size of the broadcast domain.

Consider the following example:

In the picture above we have one huge network: 10.0.0.0/24. All hosts on the network are in the same subnet, which has following disadvantages:

a single broadcast domain – all hosts are in the same broadcast domain. A broadcast sent by any device on the network will be processed by all hosts.

network security – each device can reach any other device on the subnet, which can present security problems. For example, a server containing sensitive information would be in the same network as an ordinary end user workstation.

organizational problems – in a large networks, different departments are usually grouped into different subnets. For example, you can group all devices from the Accounting department in the same subnet and then give access to sensitive financial data only to hosts from that subnet.

The network above could be subnetted like this:

Now, two subnets were created for different departments: 10.0.0.0/24 for Accounting and 10.1.0.0/24 for Marketing. Devices in each subnet are now in a different broadcast domain.

PRIVIOUS TOPIC:-CLASSES OF IP ADDRESSES

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CLASSES OF IP ADDRESSES

TCP/IP defines five classes of IP addresses: class A, B, C, D, and E. Each class has a range of valid IP addresses. The value of the first octet determines the class. IP addresses from the first three classes (A, B and C) can be used for host addresses. The other two classes are used for other purposes (class D for multicast and class E for experimental purposes).

The system of IP address classes was developed for the purpose of Internet IP addresses assignment. The classes created were based on the network size. For example, for the small number of networks with a very large number of hosts, the Class A was created. The Class C was created for the numerous networks with the small number of hosts.

                                             Classes of IP addresses are:

For the IP addresses from Class A, the first 8 bits (the first decimal number) represent the network part, while the remaining 24 bits represent the host part. For Class B, the first 16 bits (the first two numbers) represent the network part, while the remaining 16 bits represent the host part. For Class C, the first 24 bits represent the network part, while the remaining 8 bits represent the host part.

Consider the following IP addresses:

10.50.120.7 – because this is a Class A address, the first number (10) represents the network part, while the remainder of the address represents the host part (50.120.7). This means that, in order for devices to be on the same network, the first number of their IP addresses has to be the same for both devices. In this case, a device with the IP address of 10.47.8.4 is on the same network as the device with the IP address listed above. The device with the IP address 11.5.4.3 is not on the same network, because the first number of its IP address is different.

172.16.55.13 – because this is a Class B address, the first two numbers (172.16) represents the network part, while the remainder of the address represents the host part (55.13). The device with the IP address of 172.16.254.3 is on the same network, while a device with the IP address of 172.55.54.74 isn’t.

NOTE

The system of network address ranges described here is generally bypassed today by use of the Classless Inter-Domain Routing (CIDR) addressing.

Special IP address ranges that are used for special purposes are:

0.0.0.0/8 – addresses used to communicate with the current network

127.0.0.0/8 – loopback addresses

169.254.0.0/16 – link-local addresses (APIPA)

PRIVIOUS TOPIC:-TYPES OF IP ADDRESSES

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TYPES OF IP ADDRESSES

The IP addresses are divided into three different types, based on their operational characteristics: 1. unicast IP addresses – an address of a single interface. The IP addresses of this type are used for one-to-one communication. Unicast IP addresses are used to direct packets to a specific host. Here is an example:

In the picture above you can see that the host wants to communicate with the server. It uses the (unicast) IP address of the server (192.168.0.150) to do so.

2. multicast IP addresses – used for one-to-many communication. Multicast messages are sent to IP multicast group addresses. Routers forward copies of the packet out to every interface that has hosts subscribed to that group address. Only the hosts that need to receive the message will process the packets. All other hosts on the LAN will disard them. Here is an example:

R1 has sent a multicast packet destined for 224.0.0.9. This is an RIPv2 packet, and only routers on the network should read it. R2 will receive the packet and read it. All other hosts on the LAN will discard the packet.

3. broadcast IP addresses – used to send data to all possible destinations in the broadcast domain (the one-to-everybody communication). The broadcast address for a network has all host bits on. For example, for the network 192.168.30.0 255.255.255.0 the broadcast address would be 192.168.0.255. Also, the IP address of all 1’s (255.255.255.255) can be used for local broadcast. Here’s an example:

R1 wants to communicate with all hosts on the network and has sent a broadcast packet to the broadcast IP address of 192.168.30.255. All hosts in the same broadcast domain will receive and process the packet.

PRIVIOUS TOPIC:- TYPES OF ETHERNET CABLING