CCNA LESSON 7 - Subnetting

7.Subnetting

Basic of Subnetting

• We learned how to define and find the valid host ranges used in a Class A, Class B, and Class C network address by turning the host bits all off and then all on.
• You were defining only one network.
• What happens if you wanted to take one network address and create six networks from it?
• You would have to do something called subnetting, because that’s what allows you to take one larger network and break it into a bunch of smaller networks.
• Subnetting is logically dividing the network by extending the 1’s used in Subnet M ask.
• Subnetting allows you to create multiple logical networks that exist within a single Class A, B, or C network.
• If you do not subnet, you are only able to use one network from your Class A, B, or C network.

Advantage of Subnetting

• Reduced network traffic 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 which is a result of reduced network traffic.
• Simplified management It is 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.

IP Subnet- Zero

• This command allows you to use the first and last subnet in your network design.
• For example, the Class C mask of 192 provides subnets 64 and 128, but with the ip subnet- zero command, you now get to use subnets 0, 64, 128, and 192.
• That is two more subnets for every subnet mask we use.
• This router output shows that the command ip subnet-zero is enabled on the router. Cisco has turned this command on by default starting with Cisco IOS version 12.x.

P1R1# sh running-config
Building configuration...
Current configuration : 827 bytes !
hostname Pod1R1 !
ip subnet-zero !

How to Create Subnets

To create a subnet follow these steps:
1. Determine the number of required network IDs:
        • One for each subnet
        • One for each wide area network connection
2. Determine the number of required host IDs per subnet:
        • One for each TCP/ IP host
        • One for each router interface
3. 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

Classless Inter-Domain Routing (CIDR)

• Classless Inter-Domain Routing (CIDR) is basically the method that ISPs (Internet service providers) use to allocate a number of addresses to a company, a home-a customer.
• 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.

• For Class A default subnet mask 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.
• In Class B default mask would be 255.255.0.0, which is a 16 because 16 bits are ones (1s): 11111111.11111111.00000000.00000000.

A listing of every available subnet mask and its equivalent CIDR slash notation.

CIDR Value /8 to /15 can only be used with Class A network addresses.
While /16 through /23 can be used by Class A and B network addresses and /24 through /30 can be used by Class A, B, and C network addresses.
This is a big reason why most companies use Class A network addresses.
Since they can use all subnet masks, they get the maximum flexibility in network design.

Subnetting of Class C Addresses

• There are several 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 bit .
• This means that the only Class C subnet masks can be the following:
• We can’t use a /31 or /32 because we have to have at least 2 host bits for assigning IP addresses to hosts.
• After choosing 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:

1. How many subnets does the chosen subnet mask produce?
2. How many valid hosts per subnet are available?
3. What are the valid subnets?
4. What’s the broadcast address of each subnet?
5. What are the valid hosts in each subnet?

Subnetting Practice Example 1:

Class C Addresses
• Practice Example # 1C: 255.255.255.128 (/25)
• Here 128 is 10000000 in binary, there is only 1 bit for subnetting and 7 bits for hosts. We’re going to subnet the Class C network address
192.168.10.0.
192.168.10.0 = Network address
255.255.255.128 = Subnet mask

• Now, let’s answer the big five:
How many subnets?

Since 128 is 1 bit on (10000000), the answer would be 21 = 2.
How many hosts per subnet? We have 7 host bits off (10000000), so the equation would be 27 – 2 = 126 hosts.

What are the valid subnets?

256 – 128 = 128. Remember, we’ll start at zero and count in our block size, so our subnets are 0, 128.

What’s the broadcast address for each subnet?

The number right before the value of the next subnet is all host bits turned on and equals the broadcast address. For the zero subnet, the next subnet is 128, so the broadcast of the 0 subnet is 127.

What are the valid hosts?

These are the numbers between the subnet and broadcast address. The easiest way to find the hosts is to write out the subnet address and the broadcast address. This way, the valid hosts are obvious. The following table shows the 0 and 128 subnets, the valid host ranges of each, and the broadcast address of both subnets: