IPv4 Network Classes

IPv4 Network Classes

  February 28, 2014    asghars

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IPv4 addresses are divided into three groups:

Unicast Addresses

It includes Class A, B and C. Firs octet most significant bit identifies the class of IP address.

• Class A IP addresses first octet start with 1-126.The first octet has a format 0xxxxxxx,where 0 identifies that it is Class A. 
• Class B IP addresses first octet start with 128-191.The first octet has format 10xxxxxx, where 10 identifies that it is class B.
• Class C IP addresses first octet start with 192-223.The first octet has format 110xxxxx, where 110 identifies that it is class C.

Multicast Addresses

It includes Class D.

• Class D IP addresses first octet start with 224-239.

Experimental Addresses

It includes Class E. These addresses are only used for experimental purposes and its first Octet start with
240-255.

Table lists the different facts about Class A, B and C networks.

• Network Number/ID is the lowest number in the network
• Total networks are the available number of networks in one class, it can be calculated by 2^network_bits. It must be noted that network bits are always one, two and three bit less; it is because the Class A reserve 1-bit, Class B reserve 2-bits and Class C reserve 3-bits for the identification of class.
• The mask communicates the network and host parts of the IP address to the computer, therefore; every Class has its associated default subnet mask.
• Broadcast Address of a Class is the numerically highest number in network. Routers forward a copy of packet with broadcast address as destination to all hosts in Classful network.
• In between Network ID and Network Broadcast Address is the range of Usable IP Addresses. It can be calculated by 2^h-2. Two addresses (namely; network ID and broadcast address) are subtracted from the total available addresses as there addresses can't be used for hosts/nodes.

To find then network ID, Network Broadcast and Usable Address for an IP Address follow the following steps: 

1. Determine class of the IP address by examining first octet.
2. Divide the IP address into network and host part.
3. To find network ID, change host part octets to 0.
4. To find broadcast address, change network ID's host octets to 255.
5. To find first usable address, add 1 to the fourth octet of the network ID.
6. To find last useable address, subtract 1 from the fourth octet of the network broadcast address.

Example (223.223.0.1)

1. Class C
2. Network Part (223.223.0), Host Part (1)
3. Network ID = 223.223.0.0
4. Broadcast Address = 223.223.0.255
5. First Usable Address = 223.223.0.1
6. Last Usable Address = 223.223.0.254

Reserved Network IDs

• Class A network 0.0.0.0 was reserved for broadcasting requirements.
• Class A network 127.0.0.1 is reserved for loopback addresses, which is used for software testing.

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Design Stages of IPv4 Subnetting

Design Stages of IPv4 Subnetting

  February 16, 2014    asghars

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The design process of subnetting may involve the following steps:

1. Investigate the Requirements for Subnetting
2. Design your subnets to meet your requirements
3. Decide how to implement the subnets

1) Investigate the Requirements for Subnetting

In investigation stage you decide:

• Which nodes or hosts should be grouped in the same subnet?
• Next by studying network diagrams etc. decide the total number of subnets, keep in mind that VLANs also require separate subnets to operate.
• Decide the number of IP addresses each subnet will require, this will also include IP addresses for the router interfaces and switch IP addresses which are to be used for telnet, ssh etc.
• Decide to use a single subnet mask for all subnets or variable length subnet mask (VLSM) to avoid wastage of IP addresses.

2) Design your Subnets to Meet your Requirements

At design stage you will:

• Select the Internet Addressing Architecture
• Select the Subnet Mask
• Make list of all subnets to use

Internet Addressing Architecture

Internet Addressing Architecture divides the IP addresses into to two address spaces.

Private IP Addresses: Packets addresses with private IP address cannot be routed or transmitted through the Internet, however; they can be used by enterprises within their local networks. Following is the list of private IP addresses:

Class A --- 10.0.0.0 (No. of networks 1)
Class B --- 172.16.0.0 through 172.3.1.0.0 (No. of networks 16)
Class C – 192.168.0.0 through 192.168.255.0 (No. of networks 256)

Public IP Addresses: The Internet design requires that any one connecting to the Internet has to use a Public IP address, public IP addresses can only be routed through Internet. The problem face the IPv4 public IP address space is that it exhausted in 2011, to solve the issue the Internet community come-up with solutions like:

• 128-bit IPv6
• Network Address Translation (NAT); which allows the mapping of hundreds of private IP addresses to a single or more than one public IP address

Select the Subnet Mask

Once you decided on the number of subnets, host per subnet required, type of subnet and choose the Internet addressing architecture, next step would be to select the mask. Subnet mask define the network, subnet and host part of the network.

Mask selected must have enough subnet bits to number all subnets and at the same time the host part bits must also be large enough to accommodate the number of hosts in each subnet.

It’s worth mentioning here:

• That the difference between Classful and Classless Networks is that Classful network use the default mask define for the Class (e.g. Class A’s default mask is 255.0.0.0), while the Classless network use the variable length subnet mask.
• Subnet mask can be represented in decimal (255.0.0.0), binary (11111111.00000000.00000000.00000000), or prefix (/8)

Make List of All Subnets to Use

Find the subnet ID, subnet broadcast for each subnet, and IP addresses to use.

Subnet ID is the numerically the smallest number in subnet and it can’t be used as IP address for host.

Subnet Broadcast address is the numerically highest number in subnet and it is used for directed broadcast in the subnet.

IP addresses are the addresses between the subnet ID and subnet broadcast address, it can be used as host’s IP address.

3) Decide How to Implement the Subnets

Base on your network diagram identify the location for subnet, pick one from the list. Separate the static IP addresses and dynamic IP addresses assigned through DHCP.

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