IP Address

What is an IP Address?

An IP (Internet Protocol) address is a unique identifier assigned to each device connected to a network that uses the Internet Protocol for communication. This address helps to identify both the host and the location of the host within the network, allowing data to be sent and received accurately.

IP Address Structure

IP addresses are typically written in the format of four numbers separated by dots, known as dotted decimal notation. For example: 1128.11.3.31. Each number in an IP address represents 8 bits, so an IPv4 address (which is most commonly used) is a 32-bit address divided into four octets.

IP Address Classes ( Classful Addressing)

IPv4 addresses are divided into five classes (A, B, C, D, and E), primarily based on the network size they are suited for. Classes A, B, and C are commonly used for hosts, while classes D and E are reserved for special purposes.

Each of these classes has a valid range of IP addresses. Classes D and E are reserved for multicast and experimental purposes respectively. The order of bits in the first octet determines the classes of the IP address.

The class of IP address is used to determine the bits used for network ID and host ID and the number of total networks and hosts possible in that particular class. Each ISP or network administrator assigns an IP address to each device that is connected to its network.

Note: 

• IP addresses are globally managed by Internet Assigned Numbers Authority(IANA) and Regional Internet Registries(RIR).
• While finding the total number of host IP addresses, 2 IP addresses are not counted and are therefore, decreased from the total count because the first IP address of any network is the network number and whereas the last IP address is reserved for broadcast IP.

Class A

IP addresses belonging to class A are assigned to the networks that contain a large number of hosts. The network ID is 8 bits long.
The host ID is 24 bits long.

The higher-order bit of the first octet in class A is always set to 0. The remaining 7 bits in the first octet are used to determine network ID. The 24 bits of host ID are used to determine the host in any network. The default subnet mask for Class A is 255.x.x.x. Therefore, class A has a total of:2^24 – 2 = 16,777,214 host ID

IP addresses belonging to class A ranges from 0.0.0.0 – 127.255.255.255.

Class B

IP address belonging to class B is assigned to networks that range from medium-sized to large-sized networks. 

• The network ID is 16 bits long.
• The host ID is 16 bits long.

The higher-order bits of the first octet of IP addresses of class B are always set to 10. The remaining 14 bits are used to determine the network ID. The 16 bits of host ID are used to determine the host in any network. The default subnet mask for class B is 255.255.x.x. Class B has a total of: 

• 2^14 = 16384 network address
• 2^16 – 2 = 65534 host address

IP addresses belonging to class B ranges from 128.0.0.0 – 191.255.255.255.

                                

Class C

IP addresses belonging to class C are assigned to small-sized networks.

• The network ID is 24 bits long.
• The host ID is 8 bits long.

The higher-order bits of the first octet of IP addresses of class C is always set to 110. The remaining 21 bits are used to determine the network ID. The 8 bits of host ID are used to determine the host in any network. The default subnet mask for class C is 255.255.255.x. Class C has a total of:

• 2^21 = 2097152 network address
• 2^8 – 2 = 254 host address

IP addresses belonging to class C range from 192.0.0.0 – 223.255.255.255.

Class D

IP address belonging to class D is reserved for multi-casting. The higher-order bits of the first octet of IP addresses belonging to class D is always set to 1110. The remaining bits are for the address that interested hosts recognize.

Class D does not possess any subnet mask. IP addresses belonging to class D range from 224.0.0.0 – 239.255.255.255.

Class E

IP addresses belonging to class E are reserved for experimental and research purposes. IP addresses of class E range from 240.0.0.0 – 255.255.255.255. This class doesn’t have any subnet mask. The higher-order bits of the first octet of class E are always set to 1111.

Range of Special IP Addresses

169.254.0.0 – 169.254.0.16 : Link-local addresses
127.0.0.0 – 127.255.255.255 : Loop-back addresses
0.0.0.0 – 0.0.0.8: used to communicate within the current network.

Rules for Assigning Host ID

Host IDs are used to identify a host within a network. The host ID is assigned based on the following rules:

• Within any network, the host ID must be unique to that network.
• A host ID in which all bits are set to 0 cannot be assigned because this host ID is used to represent the network ID of the IP address.
• Host ID in which all bits are set to 1 cannot be assigned because this host ID is reserved as a broadcast address to send packets to all the hosts present on that particular network.

Rules for Assigning Network ID

Hosts that are located on the same physical network are identified by the network ID, as all host on the same physical network is assigned the same network ID. The network ID is assigned based on the following rules:

• The network ID cannot start with 127 because 127 belongs to the class A address and is reserved for internal loopback functions.
• All bits of network ID set to 1 are reserved for use as an IP broadcast address and therefore, cannot be used.
• All bits of network ID set to 0 are used to denote a specific host on the local network and are not routed and therefore, aren’t used.

Summary of classful addressing

Note: in class A number of network is 127.

1. Subnetting Overview

Subnetting divides a larger IP network into smaller, manageable segments (subnets). This is useful for organizing network devices logically, improving security, and conserving IP addresses.

Each subnet functions as an isolated network with its own range of IP addresses, and this setup allows devices within a subnet to communicate directly while restricting external access.

Why Subnetting?

• Efficient Use of IP Addresses: Subnetting helps to allocate the appropriate number of IP addresses for each network segment, preventing waste.
• Improved Network Management: It allows administrators to organize devices logically, such as by department or geographical location.
• Enhanced Security: Traffic within a subnet can be isolated from other subnets, protecting internal communications.
• Reduced Broadcast Traffic: Smaller networks reduce the volume of broadcast traffic, which can improve network performance.

2. Subnet Masks

A subnet mask is used to identify the network and host portions of an IP address. It essentially "masks" the IP address to distinguish the parts used for network identification from those used for hosts.

For example:

• Subnet Mask for Class A: 255.0.0.0
• Subnet Mask for Class B: 255.255.0.0
• Subnet Mask for Class C: 255.255.255.0

These default masks can be adjusted to create smaller subnets, allowing us to control the number of subnets and the number of hosts per subnet.

3. CIDR Notation

Classless Inter-Domain Routing (CIDR) notation simplifies the process of subnetting by allowing us to use a "prefix length" to indicate how many bits are used for the network portion. This notation is written as an IP address followed by a / and the number of bits used for the network portion.

For example:

• 192.168.1.0/24: This CIDR notation means the first 24 bits are the network portion, and the last 8 bits are available for host addresses within this subnet. This is commonly used for Class C addresses.
• 172.16.0.0/16: Here, the first 16 bits are the network portion, and the remaining bits are for hosts, typically used for Class B addresses.

4. Subnetting Example

Let’s look at how to create subnets within a Class C network using CIDR notation.

Suppose we have the network 192.168.1.0/24. By default, this subnet mask (/24) allows for 256 IP addresses (from 192.168.1.0 to 192.168.1.255), with 254 usable addresses for hosts.

Example: Dividing 192.168.1.0/24 into Smaller Subnets

If we want to divide this network into smaller segments (e.g., four subnets), we can extend the subnet mask by 2 more bits, creating a /26 mask (which uses the first 26 bits for the network portion).

• Subnet Mask: 255.255.255.192 or /26
• Number of Hosts per Subnet: Each /26 subnet has 64 IP addresses, but 62 are usable for hosts (as the first address is the network address and the last is the broadcast address).

These four subnets would be:

1. 192.168.1.0/26 - Host range: 192.168.1.1 to 192.168.1.62
2. 192.168.1.64/26 - Host range: 192.168.1.65 to 192.168.1.126
3. 192.168.1.128/26 - Host range: 192.168.1.129 to 192.168.1.190
4. 192.168.1.192/26 - Host range: 192.168.1.193 to 192.168.1.254

This way, we have split the original Class C network into four subnets, each with 62 usable IP addresses.

5. Key Points for Subnetting with CIDR Notation

• Bits Used in Subnet Mask: The more bits we allocate to the network portion (the higher the / number), the fewer hosts each subnet can support.
• Subnet Calculation Formula:
  • Total Number of Subnets = $2^{\text{number of subnet bits}}$
    
  • Total Hosts per Subnet = $2^{\text{number of host bits}} - 2$ (Subtracting 2 for the network and broadcast addresses)

Using these formulas, you can create customized subnets based on the specific needs of your network.