Subnetting

Subnetting

The process of taking a large network and splitting it up intro many individual amd small subnetworks, or subnets.

Incorrect subnetting setups are a common problem you might run into as an IT Support Specialist, so it's important to have a strong understanding of how this works.

Subnet Masks

Network ID, Host ID and Subnet ID

Subnet Masks

32-bit numbers that are normally written out as four octets in decimal

The purpose of "masks", or the all 1s is to tell the router what part of an IP address is the subnet ID. The 0s mean the amount of possible host ID addresses.

Th size of the subnet ID is entirely defined by the subnet mask.

number 0 is generally not used and number 255 is normally reserved for broadcast address for the subnet. 1-254 is available for assignment to the host.

If subnet mask is 255.255.255.224, in binary (11111111. 11111111. 11111111. 11100000). This means that we have 5 bit of host ID spaces, or total of 32 addresses. It is also referenced as "/27" as the number of 1s. (e.g. 9.100.100.100/27)

Subnet Mask

A way for a computer to use AND operators to determine if an IP address exists on the same network

IP address : 9.100.100.100

Subnet Mask : 255.255.255.0

(Do AND operation between octets)

= Network ID and Subnet ID portion of IP address: 9.100.100

Compare the result with its own network ID to determine if the address is on the same network or not.

CIDR

Address Class - First attempt to split up global internet IP spaces.

Subnetting - Address class wasn't sufficient enough.

Network ID (8 bit - Class A, 16 bit - Class B, 24 bit - Class C)

CIDR (Classless Inter-Domain Routing)

Many companies ended up with various adjoining Class C networks to meet their needs. Routing tables ended up with a bunch of entries for a bunch of Class C networks that were all actually being routed to the same place. In this context gave birth to CIDR.

Even more flexible approach to describing blocks of IP addresses.

It expends on the concepts of subnetting by using subnet maskes to demarcate (to set something off) networks.

Demarcation Point

To describe where one network or system ends and another one begins.

With CIDR, network ID and subnet ID are combined into one. The slash notation of subnet mask is also known as CIDR notation.

CIDR abandons the concept of Address Class entirely.

CIDR allows for the networks themselves to be different in sizes.

Instead of using 2 Class C, combine that address space into one contiguous chunk with the netmask of 23(255.255.254.0). We also get additional host IDs this way.

/24 network is 8 host bits. 2⁸ = 256

256 - 2 = 254

254 + 254 = 508

/23 network is 9 host bits. 2⁹ = 512

512 - 2 = 510