Subnetmask :

          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 accomolished 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 address.

       The default subnet masks for Classes A,BandC. These default masks cannot change. These default mask cannot change. In other words, you can't make a ClassB subnet mask read 255.0.0.0. If you try, the will read that address as invalid and usually won't even let u type it in like wise Class A, Class C also.

     

Class	Format	Default Subnet Mask
A	network . node . node . node	255.0.0.0
B	network . network . node . node	255.255.0.0
C	network . network . network . node	255.255.255.0

Subnetting Basic

    To find the valid host 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. This is very good, but here's the catch: You were defining only one network address create six networks form it? you would have to to do something called subnetting, because that's what allows you to take one larger network and break in into a bunch of smaller networks. there are loads of reasons in favor of subnetting, including the following benefits:

• Reduced network traffic We all appreciate less traffic of any kind. Network 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.

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 255.255.255.192 provides subnets  64 and 128. But with IP subnet-zero command, you now get to use subnets 0,64,128and 192. This command works in CISCO routers.

router(config)# ip subnet-zero. this works on 1800 series and c2600-adventerprisek9_ivs-mz.124-17 IOS version.

How To create subnet To create subnets, 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.

Classless inter-Domain Routing(CDIR) Another term you need to familiarize yourself with is classless inter-domain routing (CDIR). It's basically the method that ISP's(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 look like something this:192.168.10.32/28. This telling you what your subnet mask is. The slash notation (/) means how many bits are turn as 1's . Obviously the maximum could only be /32 because a byte is 8 bits and there are 4 bytes in an IP address:(4x8=32). But the largest subnet mask available can only be a /30 because uoy got to keep atleast 2 bits for host bits. Example: A class B default MASK would be 255.255.0.0 which is /16 because 16 bits are ones i.e., 11111111.11111111.00000000.00000000 this was in binary format.

Subnet Mask	CIDR Value
255.0.0.0	/8
255.128.0.0	/9
255.192.0.0	/10
255.224.0.0	/11
255.240.0.0	/12
255.248.0.0	/13
255.252.0.0	/14
255.254.0.0	/15
255.255.0.0	/16
255.255.128.0	/17
255.255.192.0	/18
255.255.224.0	/19
255.255.240.0	/20
255.255.248.0	/21
255.255.252.0	/22
255.255.254.0	/23
255.255.255.0	/24
255.255.255.128	/25
255.255.255.192	/26
255.255.255.224	/27
255.255.255.240	/28
255.255.255.248	/29
255.255.255.252	/30

Example: 

 Subnetting Class C addresses There are many different 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 submit bits start at the left and go to the right, without skipping bits. This means that the only Class C masks can be shown in the table

Binary	Decimal	CIDR
00000000	0	/24
10000000	128	/25
11000000	192	/26
11100000	224	/27
11110000	240	/28
11111000	248	/29
11111100	252	/30

We can't use a /31 or /32 because we have to have at least 2 host bits for assigning IP address to hosts. In the past, now discussed the /25 in a Class C network. In Cisco always had been concerd with having at least2 subnet bits, but now, because of Cisco recognizing the ip subnet-zero command in its curriculum.

Subnetting Class B Address

Let's look at all the possible Class B subnet mask first.

255.255.0.0 (/16)

255.255.128.0 (/17)

255.255.192.0 (/18)

255.255.224.0 (/19)

255.255.240.0 (/20)

255.255.248.0 (/21)

255.255.252.0 (/22)

255.255.254.0 (/23)

255.255.255.0 (/24)

255.255.255.128 (/25)

255.255.255.192 (/26)

255.255.255.224 (/27)

255.255.255.240 (/28)

255.255.255.248 (/29)

255.255.255.252 (/30)

We know that 16 bits available for host addressing. This means we can use up to 14 bits for subnetting. The process of subnetting a Class B network is pretty much the same as it is for a class C, expect that you just have more host bits and you start in the third octet.

Subnetting Class A address Class A subnetting is not performed any differently than Classes B and C, but there are 24 bits with inserted of the 16 in a Class B address and the 8 in a class C address.

255.0.0.0 (/8)

255.128.0.0 (/9)

255.192.0.0 (/10)

255.224.0.0 (/11)

255.240.0.0 (/12)

255.248.0.0 (/13)

255.252.0.0 (/14)

255.255.0.0 (/15)

255.255.0.0 (/16)

255.255.128.0 (/17)

255.255.192.0 (/18)

255.255.224.0 (/19)

255.255.240.0 (/20)

255.255.248.0 (/21)

255.255.252.0 (/22)

255.255.254.0 (/23)

255.255.255.0 (/24)

255.255.255.128 (/25)

255.255.255.192 (/26)

255.255.255.224 (/27)

255.255.255.240 (/28)

255.255.255.248 (/29)

255.255.255.252 (/30)

Variable Length Subnet Masks(VLSM) For Example take one network and create many networks using subnet masks of different lengths on different types of network designs. This is called VLSM networking.

           neither RIP v1 nor IGRP routing protocols have a field for subnet information, so the subnet information gets dropped. What this means is that if a router running RIP has a subnet mask of a certain value, it assumes that all interfaces within the classful address space have the same subnet mask. This is called classful routing, and RIP and IGRP are both considered classful routing protocols. If you mix and match subnet mask length in a network running RIP or IGRP, that network just won't work.

           Classless routing protocols, however, do support the advertisement of subnet information. Therefore, you can use VLSM  with routing protocols such as RIPv2,EIGRP and OSPF. The benfit of this type of network is that you save a bunch of IP address space with it. As the name suggests, with VLSMs we can have different length subnet masks for different router interfaces.

VLSM Design A classesless design which will become the new network.In a LAN didn't have enough address because every router interface and host used the same subnet mask. Not so good. What would be good is to provide only the needed number of hosts on each router interface. To do this, we use what are referred to as variable length Subnet Masks(VLSMs).

    Now remember that we can use different size masks on each router interface. And if we use 0/30 on our WAN links and a /27,/28,and /29 on our LANs, we'll set 2 hosts per WAN interface, and 30,14, and 8 hosts per LAN interface. This makes a huge difference not only can we get just the right amount of hosts on each LAN, we still have room to add more WANs and LANs using this same network.

Implementing VLSM Networks To create VLSMs quickly and efficiently, you need to understand how block sizes and charts work together to create the VLSM masks. I shown you the block sizes used when creating VLSMs with class C networks. For example, if you need 25 hosts, then you'll need a lock size of 32. If you need 11 hosts, you'll use a block size of 16. Need 40 hosts? then you'll need 64 block. you cannot just makeup block sizes they have got to be the blocksizes. 

Prefix	Mask	Hosts	Block size
/25	128	26	128
/26	192	62	64
/27	224	30	32
/28	240	14	16
/29	248	6	8
/30	252	2	4