Subnetting Basics - Self Learn


To understand IP address resolution, you need to understand how to decode the IP address and extract the two main identifiers:
  • A network ID (the network bits)
  • A host ID (the bits the system administrator controls locally when assigning IP addresses)

Network ID is used to identify the number of  networks and Host ID is used to identify the number of hosts on the local-area network.


For now, just keep in mind a few subnetting basics:
  • A subnet mask — or subnet address — is used by routers to assist in determining the network portion of an IP address from the host portion.
  • The subnet mask is a 32-bit value just like the IP address. The binary 1 bits correspond to the network and subnet portions of the address. The 0 bits are host representatives.
  • Three standard masks exist for Class A, B, and C networks:
    • Standard Class A subnet mask: 255.0.0.0
    • Standard Class B subnet mask: 255.255.0.0
    • Standard Class C subnet mask: 255.255.255.0


Example : 172.168.16.100
Image from book 
Remember IP addresses with decimal 0s in the host address octets represent the network segment and are reserved (not issued to network clients).
I now take a look at network and host addressing using the previous IP address example of 172.16.10.100. You already know that 172.16 is the network portion and 10.100 is the host portion of this Class B network address. The entire 172.16 network is specified using 0s, or written as 172.16.0.0, and is not assignable to any network client. This IP represents the entire 172.16 network "wire" and is reserved. The 172.16 network can be subdivided or split into subnetworks, or subnets, using, or "borrowing," from the host portion of the IP address as follows:
172.16.1.0
Subnet 1 on network 172.16.0.0
172.16.2.0
Subnet 2 on network 172.16.0.0
172.16.3.0
Subnet 3 on network 172.16.0.0
172.16.16.0
Subnet 16 on network 172.16.0.0

Click to collapse

Purpose of Subnetting

One of the main reasons for subnetting is to split one large network into smaller, logical chunks of space. This makes things easier to manage and also keeps problems restricted to their own subnets. Broadcast traffic is minimized throughout the network and is contained in each group's network structure. This helps significantly with troubleshooting issues and isolating problems on the network.
Another good reason for subnetting is having two portions of the network that are geographically separated from one another. Each remote site connects using a router's WAN link and uses separate network IDs in the IP address to split them logically. They still belong to the same overall corporate network but are divided into subnets for management and location-related reasons. 
Click to collapse


Borrowing from the network ID to create subnets.
Click to collapse

Subnet Masks

Subnet masking is used to recognize or reveal the subnet contained in the IP address.
Every node on the same subnet must have the same subnet mask. 
The subnet mask helps you to decode the IP address and decipher the individual network bits, subnet bits, and host 
— or interface — bits.
 Network devices use the subnet mask to determine which part of the IP is a network address and which part is the interface. By using the IP address and subnet mask, networking devices can calculate which devices are located on the same subnet, on a different subnet on the same organizational network, or on a completely different network.
Click to collapse

subnet are used only in local area network and used Host ID portion in the IP address.

Subnet Mask, Network ID, Host ID, and Broadcast IP

I first review the relationship among subnet masks, network and host IDs, and the broadcast IP address for a given subnet by examining the following example.
Assume that you have a network address of 172.16.0.0 with the default subnet mask of 255.255.0.0. You then "borrow" from the host portion (host portions are the third and fourth octet) or shift over 1 complete byte of the subnet mask, extending the network portion of the subnet mask to cover the entire third octet. The subnet mask would then look like this: 255.255.255.0. Your change looks like this in binary:
  • 11111111.11111111.00000000.00000000
  • to
  • 11111111.11111111.11111111.00000000
The first 16 bits in both examples define the original network ID. The change comes in the third octet, which has now morphed from a host ID octet to a subnetted one. The fourth octet is now used for hosts using the new mask compared to the third and fourth octets from the standard mask.
You have just created an 8-bit subnet by changing the default mask from 255.255.0.0 to 255.255.255.0. What does this give you for choices when planning your organizational subnet structure? Discounting the reserved network segment — or "wire" — dotted-decimal address of 0 and the reserved broadcast decimal address of 255, you can determine that you now have the possibility of assigning 254 unique host addresses on 256 unique subnets.
So for instance, subnet 172.16.1.0 has the host range possibilities of 172.16.1.1–172.16.1.254, with 172.16.1.255 reserved as the broadcast address for that subnet. I examine typical examples of Class A, B, and C subnetting in an upcoming section, but for now, I turn my attention to classless interdomain routing.

Classless Interdomain Routing (CIDR)

CIDR uses a masking technique (instead of a classful addressing one) to determine the target network, eliminating the classful configuration limitations and preventing the waste of classful system addressing. In classful addressing, the amount of assignable host and network addresses available to the system administrator can be excessive and prove to be very inefficient.
Click to collapse

In the previous IP address examples, you can determine which class an IP address belongs to by categorizing it in one particular class list (A, B, or C). With CIDR notation, this is no longer the case. You cannot identify a particular class by examining the octets alone. You must use a modified method of examining the subnet mask. In CIDR notation, you follow the IP address with the amount of bits assigned as the network ID, often called the prefix length. This CIDR, or slash notation, is written as follows:
  • 180.10.150.0/22
  • 180.10.150.0 255.255.252.0
Click to collapse


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