y Chapter 5, Subnetting

y Chapter 5, Subnet Overview

y 1. Subnets

A. A subnet is a physical segment in a TCP/IP environment that uses IP addresses derived from a single network ID.

B. Each segment uses a different network ID or subnet ID.

C. Subnetting offers several advantages.

1. Allows you to mix different technologies such as Ethernet and Token Ring

2. Allows you to overcome limitations of current technologies, such as number of hosts per segment

3. Allows you to reduce network congestion by redirecting traffic and reducing broadcasts

y 2. Implementing Subnetting

A. Use these guidelines for determining subnet requirements.

1. Determine the current and future number of physical segments on your network.

2. Determine the current and future number of host addresses required for each physical segment.

3. Based on these requirements, define one subnet mask for the entire network, a unique subnet ID for each physical segment, and a range of host IDs for each subnet.

y B. Subnet Mask Bits

1. Subnet mask bits are taken from the host bits available for each class of address.

2. Each bit taken for subnetting reduces the number of host bits available for host addresses per subnet.

3. Summary

A. A subnet is a physical segment in a TCP/IP environment that uses IP addresses from a single network ID and borrows host ID bits for use as a subnet ID.

B. The number of bits in the subnet mask will determine the number of subnets and hosts per subnet available to you.

Chapter 5, Defining a Subnet Mask

y 1. Defining a Subnet Mask

A. Use these guidelines for defining a subnet mask.

1. Convert the required number of physical segments in your network to binary format.

2. Count the number of bits required to represent the number in binary. For example, decimal 4 converted to binary is 100 and requires 3 bits.

3. Convert the required number of bits to decimal format in high order (from left to right). For example, decimal 4 requires 3 bits; therefore, configure the first 3 bits of the host ID as the subnet ID. The decimal value for 11100000 is 224. For a class B address, the subnet mask would be 255.255.224.0.

B. According to the original RFC, subnet mask bits do not technically have to be contiguous. Some vendors use low-order bits. It is now a requirement that the subnet ID make use of contiguous high-order bits of the local address portion of the subnet mask.

y 2. Subnetting More Than One Octet

A. It is possible to use more than one octet for subnetting. For example, if a company has a Class A address and needs to use 13 bits for subnetting, it will use a subnet mask of 11111111.11111111.11111000.00000000 or 255.255.252.0.

See my page on Simplified Subnetting to help explain how to apply subnetting principles.

Chapter 5, Supernetting

y 1. Supernetting

A. Supernetting borrows bits from the network ID and masks them as the host ID.

B. Supernetting combines several Class C addresses which each accommodate 254 hosts to accommodate larger numbers of hosts.

C. Classless Inter-Domain Routing (CIDR) is used to collapse multiple Class C addresses into one network ID.

D. Supernetting reduces entries in routing tables.

2. Summary

A. Subnetting borrows bits from the host ID portion of a regular Class A, B, or C address for use in creating subnets that actually increase the number of network IDs within a company.

B. Supernetting combines multiple network IDs to create a “super ID” that will support more hosts than one of the network IDs could support.

See my page on Simplified Supernetting to help explain how to apply supernetting principles.

y Chapter Review