SUBNETTING

Benefits of subnetting

Reduced network traffic

One network will not access the data of other network without the use of router. Thus we can reduce the amount of data remain in one network. Less data less overhead, collision, or broadcast storm.

Optimized network performance

This is a result of reduced network traffic.

Simplified management

It's easier to identify and isolate network problems in a group of Smaller connected networks than within one gigantic network. Facilitated spanning of large geographical distances Because WAN links are significantly slower and more expensive than LAN links, a single large network that spans long distances can create problems in every area earlier listed. Connecting multiple smaller networks makes the system more efficient.

Powers of 2

Powers of 2 are important to understand and memorize for use with IP subnetting.

21 2 29 512 22 4 210 1024 23 8 211 2048 24 16 212 4096 25 32 213 8192 26 64 214 16384 27 128 215 32768 28 256 216 65536

Before we go further let’s get familiar with subnetting components

Subnet mask

A subnet mask is a 32-bit value that allows the receiver of IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address. Every IP address is composed of a network component and a host component. The subnet mask has a single purpose: to identify which part of an IP address is the network component and which part is the host component. Subnet mask value 0 represent host ID while subnet mask value 1 to 255 represents Network ID in ip address.

Classless Inter-Domain Routing (CIDR)

This slash notation is sometimes called CIDR (Classless Inter-Domain Routing) notation. It’s basically the method that ISPs (Internet service providers) use to allocate a number of Addresses to a company, a home—a customer. The slash notation is simply the number of 1s in a row in the subnet mask. The real reason to use CIDR notation is simply that it is easier to say and especially to type.

Address Class and Default Mask

Subnetting happens when we extend the subnet mask past the default boundary for the address we are working with. So it's obvious that we first need to be sure of what the default mask is supposed to be for any given address. When faced with a subnetting question, the first thing to do is decide what class the address belongs to. And later decide what the default subnet mask is. One of the rules that Cisco devices follow is that a subnet mask must be a contiguous string of 1s followed by a contiguous string of 0s. There are no exceptions to this rule: A valid mask is always a string of 1s, followed by 0s to fill up the rest of the 32 bits. (There is no such rule in the real world, but we will stick to the Cisco rules here—it's a Cisco exam, after all.) Therefore, the only possible valid values in any given octet of a subnet mask are 0, 128, 192, 224, 240, 248, 252, 254, and 255. Any other value is invalid.

Block Size

The process of subnetting creates several smaller classless subnets out of one larger classful . The spacing between these subnets, or how many IP addresses apart they are, is called the Block Size.

Network ID and Broadcast ID

The first address in a network number is called the network address, or wire number. This address is used to uniquely identify one segment or broadcast domain from all the other segments in the network.

The Broadcast ID
The last address in the network number is called the directed broadcast address and is used to represent all hosts on this network segment. it is the common address of all hosts on that Network ID. This should not be confused with a full IP broadcast to the address of 255.255.255.255, which hits every IP host that can hear it; the Broadcast ID hits only hosts on a common subnet. A directed broadcast is similar to a local broadcast.
The main difference is that routers will not propagate local broadcasts between segments, but they will, by default, propagate directed broadcasts.

Host Addresses

Any address between the network address and the directed broadcast address is called a host address for the segment. You assign these middle addresses to host devices on the segment, such as PCs, servers, routers, and switches.