Reduced network traffic: We all appreciate less traffic of any kind. Networks are no different. Without trusty routers, packet traffic could grind the entire network down to a near standstill. With routers, most traffic will stay on the local network; only packets destined for other networks will pass through the router. Routers create broadcast domains. The more broadcast domains you create, the smaller the broadcast domains and the less network traffic on each network segment.
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 considerably
slower and more expensive than LAN links, a single large network that spans long distances
can create problems in every area previously listed. Connecting multiple smaller networks
makes the system more efficient.
How to Create Subnets
To create subnetworks, you take bits from the host portion of the IP address and reserve them to define the subnet address. This means fewer bits for hosts, so the more subnets, the fewer bits available for defining hosts.To create a subnet follow these steps:
- Determine the number of required network IDs:
- One for each subnet.
- One for each wide area network connection
- Determine the number of required host IDs per subnet:
- One for each TCP/IP host
- One for each router interface
- Based on the above requirements, create the following:
- One subnet mask for your entire network
- A unique subnet ID for each physical segment
- A range of host IDs for each subnet
Subnet Masks
For the subnet address scheme to work, every machine on the network must know which part of the host address will be used as the subnet address. This is accomplished by assigning a subnet mask to each machine. A subnet mask is a 32-bit value that allows the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address.The network administrator creates a 32-bit subnet mask composed of 1s and 0s. The 1s in the subnet mask represent the positions that refer to the network or subnet addresses.
Not all networks need subnets, meaning they use the default subnet mask. This is basically the same as saying that a network doesn’t have a subnet address.
Classless Inter-Domain Routing (CIDR)
Another term you need to familiarize yourself with is Classless Inter-Domain Routing (CIDR)It’s basically the method that ISPs (Internet service providers) use to allocate a number of
addresses to a company, a home—a customer. They provide addresses in a certain block size.
When you receive a block of addresses from an ISP, what you get will look something like
this: 192.168.10.32/28. This is telling you what your subnet mask is. The slash notation (/)
means how many bits are turned on (1s). Obviously, the maximum could only be /32 because
a byte is 8 bits and there are 4 bytes in an IP address: (4 × 8 = 32). But keep in mind that the
largest subnet mask available (regardless of the class of address) can only be a /30 because
you’ve got to keep at least 2 bits for host bits.
Take, for example, a Class A default subnet mask, which is 255.0.0.0. This means that the
first byte of the subnet mask is all ones (1s), or 11111111. When referring to a slash notation,
you need to count all the 1s bits to figure out your mask. The 255.0.0.0 is considered a /8
because it has 8 bits that are 1s—that is, 8 bits that are turned on.
A Class B default mask would be 255.255.0.0, which is a /16 because 16 bits are ones (1s):
11111111.11111111.00000000.00000000.
Subnetting Class C Addresses
There are many different ways to subnet a network. The right way is the way that works best for you. In a Class C address, only 8 bits are available for defining the hosts. Remember that subnet bits start at the left and go to the right, without skipping bits. This means that the onlyClass C subnet masks can be the following:
Binary Decimal CIDR
00000000 = 0 /24
10000000 = 128 /25
11000000 = 192 /26
11100000 = 224 /27
11110000 = 240 /28
11111000 = 248 /29
11111100 = 252 /30
When you’ve chosen a possible subnet mask for your network and need to determine the number of subnets, valid hosts, and broadcast addresses of a subnet that the mask provides, all you need to do is answer five simple questions:
- How many subnets does the chosen subnet mask produce?
- How many valid hosts per subnet are available?
- What are the valid subnets?
- What’s the broadcast address of each subnet?
- What are the valid hosts in each subnet?