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An Subnetpart that is used by internal routers within the organization to deliver packets to a particular smaller Subnet. An IP Hostpart that identifies a particular individual device.TDC 594
Subnetting Example Based on Subnet RequirementsBrewCo was allocated IP address block 22.214.171.124/24 by their ISP. They have 6 departments and want to use 8 subnets internally. What Subnet Mask should they use?Initial subnet = 126.96.36.199 / 24Initial prefix, subnet mask = /24 or 255.255.255.0Size of initial subnet = 2(32-24) = 28 = 256 IP addressesNumber of new subnets = 8 = 23New prefix length = 24 + 3 = / 27New subnet mask = 255.255.255.224Size of each new subnet = 2(32-24-3)= 25 = 32Result: Original block of 256 addresses is split into 8 subnets of 32 IP addresses each.
Listing New SubnetsOriginal subnet = 188.8.131.52/24. Using mask 255.255.255.224, the 8 new subnetsare:Subnet 184.108.40.206 /27Valid host range = 220.127.116.11 – 18.104.22.168Subnet broadcast address = 22.214.171.124Subnet 126.96.36.199 /27Valid host range = 188.8.131.52 – 184.108.40.206Subnet broadcast address = 220.127.116.11Subnet 18.104.22.168 /27Valid host range = 22.214.171.124 – 126.96.36.199Subnet broadcast address = 188.8.131.52(… leaving four subnets out here …)Subnet 184.108.40.206 /27Valid host range = 220.127.116.11 – 18.104.22.168Subnet broadcast address = 22.214.171.124
Determining the Subnet MaskSubnetting Based on Subnet or Host RequirementsThere are two considerations when planning subnets:Number of Subnets requiredNumber of Host addresses requiredFormula to determine number of useable hosts 2n-22n(where n is the number the number of host bits remaining) is used to calculate the number of hosts-2 Subnetwork ID and broadcast address cannot be used on each subnet
Subnetting Example Based on Host RequirementsJoyCo Corp. has been allocated IP network 126.96.36.199/16 and is split into departments that may have up to 1000 computers each. How many equal-sized subnets can they create with at least 1000 valid host addresses in each? What subnet mask should they use?What are the resulting Subnet IDs?TDC 594
JoyCo ExampleSubnet sizes must be powers of 2. The smallest power of 2 that is greater than 1000 is 210= 1024. This would give them 1024 – 2 = 1022 valid host addresses in each subnet. Thus, each IP address needs 10 Host bits.The new prefix length will be 32 – 10 = /22.So, new Subnet mask is 255.255.252.0.The difference between old prefix length (16) and new prefix length (22) is 22-16 = 6, so 6 bits are borrowed:Number of new subnets = 26 = 64 subnetsWe also could have figured this as 65,536 / 1024 = 64TDC 594
Calculating JoyCo SubnetsFirst Subnet ID will be the original network address with the new prefix length.First Subnet ID is 188.8.131.52 / 22New Subnet Mask = 255.255.252.0Two ways to calculate other subnet IDs:Start with first subnet ID and add 1024 IP addresses to it. This will increase value in 3rdbyte by 1024 / 256 = 4.