IP Subnetting Overview

Study Notes

IP Subnetting logically divides a large network into smaller networks. This improves management, performance, and security.
Subnetting creates multiple networks requiring routers to connect them.

Reduced Network Traffic: Routers limit broadcast domains, restricting traffic to the local network unless transfer is needed outside. This reduces unnecessary traffic through routers.
Optimized Network Performance: Reduced traffic leads to improved network performance.
Simplified Management: Smaller networks make identifying and isolating problems easier.
Facilitated Spanning of Large Geographical Distances: Connecting multiple smaller networks (using WAN links) improves system efficiency compared to using more expensive and slower LAN links.

Subnets borrow bits from the host portion of the IP address to define the subnet address.
Borrowing host bits results in fewer host bits, more subnets, and fewer host addresses.
Planning current and future network requirements is crucial before implementing subnetting.

Determine the number of required network IDs (one per subnet and one per WAN connection).
Determine the number of required host IDs per subnet (one per TCP/IP host and one per router interface).
Based on the requirements, create: a subnet mask for the entire network, unique subnet IDs for each physical segment, and host ID ranges for each subnet.

A subnet mask is a 32-bit value that helps determine the network ID and host ID portions of an IP address.
Subnet masks are assigned to each network device along with the IP address. Devices without assigned masks use the default mask assigned to their IP address class.

CIDR is used by ISPs to allocate IP addresses to companies, homes, and other customers.
IP addresses are provided in a block format, like 192.168.10.32/28. The slash notation /28 indicates how many bits in the subnet mask are set to 1.
The largest possible subnet mask is /30, as at least two bits are reserved for host addresses.

Class C addresses have 8 bits reserved for hosts.
Subnetting borrows bits from the host portion, starting from the most significant bit (leftmost).
Examples of Class C subnets (using binary):
- 00000000 = 0/24
- 10000000 = 128/25
- 11000000 = 192/26
- 11100000 = 224/27
- 11110000 = 240/28
- 11111000 = 248/29
- 11111100 = 252/30

Determine the subnet mask's effect on the number of subnets and hosts per subnet.
Define the valid subnets.
Define the broadcast address for each subnet.
Determine the valid hosts for each subnet.
Number of Subnets: 2^x, where x is the number of borrowed bits.
Hosts per subnet: 2^y − 2, where y is the remaining host bits after borrowing.
Valid Subnets: 256 minus the subnet mask.
Broadcast Address: The address before the next subnet.
Valid Hosts: Numbers between the subsequent subnets, excluding network and broadcast IDs.

Subnets of Example addresses (192.168.10.0/25, 255.255.255.224/27, 255.255.255.248/29) require detailed calculation.

Subnetting Class B networks mirrors Class C, with more available host bits, starting from the third octet.
For Class B, use the same third octet subnet numbers as used for Class C, adding a zero to the network portion of the fourth octet and 255 to the broadcast address portion.

Subnetting Class A addresses is similar to Class B and C, but subnetting begins in the second octet.
Subnets are formed by appending zeros to the third and fourth octets.
Broadcast addresses are obtained by appending 255 to the third and fourth octets.