Easy lesson to Subnetting By XavierYZ Sub netting is required when you have a large network of computers! If we have a large network of computers and chose to use the default subnet of a private range what happens is the network broadcasts its entire request across the network. This creates a decrease performance. So what subnet masks allow us to do is break a network into segments, but still allow the entire network to function as one. Breaking down subnet into binary Class A 255.0.0.0 = 11111111.00000000.00000000.00000000 Class B 255.255.0.0 =11111111.11111111.0000000.00000000 Class C 255.255.255.0 =11111111.11111111.11111111.00000000 /24 are the CIDR -- Classless InterDomain Routing. CIDR was invented several years ago to keep the Internet from running out of IP addresses. E.g. 192.168.1.1/24 is the default for IP/Subnet for a class C network /24 represents Bits or 1’s…we should all know that a Class C subnet is 255.255.255.0, but how do you figure this out. We know that there are 8bits to each of the 4 octets of an IP address 11111111.11111111.11111111. < 24 1’s represent /24 of the network side Remaining octet will have all zeros which is the host side of a subnet > .00000000 *Note Subnets Start with 1’s and end in 0’s, it will never look like this 11110111.01010000.00000000.00000000 *This chart is hard to read~on the forum, I will post the html when I upload the file at home 192 224 240 248 252 254 255 128 64 32 16 8 4 2 Small numbers represent sums of each bit of an octet e.g., 11111000 = 128+64+32+16+8+0+0+0 or because there are 5 1’s we can just look at the chart and know that 5 1’s are 248 EG. 192.168.1.1/17 = /18 1’s at the front of our subnet Binary format: 11111111.111111111.11000000.00000000 Decimal format: 255.255.192.0 Now what does this subnet mean in reality? Going back to the binary format 11111111.111111111.11000000.00000000 1’s represent the number of networks you can have 0’s represent the number of hosts per network to determine the number of hosts we use this formal 2 to the power of x - 2 = hosts X = #of 0’s 2 to the power of 14 - 2 = 16382 *We subtract 2 because we don’t use 0 or 255 in the last subnet of an IP address So with an IP of 192.168.1.0/18 the subnet is 255.255.192.0 This network can have 16382 computers in one segment of a network. EG scenario. You are the network administrator of a large network using TCP/IP as its only transport protocol. You need to design a network that will handle 25 subnets with a maximum of 2000 hosts. What subnet will you use? A: 255.255.255.0 B: 255.255.240.0 C: 255.255.248.0 D: 255.255.240.0 To determine the answer use our host formula 2 to the power of x-2 = hosts X = #of 0’s We know that hosts = 2000 We need to determine how many 0’s in our subnet we need if x = 0’s Find out how many powers of 2 it takes to reach a maximum of 2000 hosts! 2 to the power of 11 - 2 =2046 Ok! Now we need to put this into a subnet mask. We now know that 11 0’s are need in our subnet mask 0’s represent Hosts 32bits (0’s and 1’s) – 11 bits (0’s) = 21 1’s 11111111.11111111.11111000.00000000 this is our binary format, Transfer it to decimal form and you have 255.255.248.0 So to answer this question A subnet mask of 255.255.248.0 or 192.168.1.0/21 will allow us to have at least 2046 hosts. If anyone finds this txt format difficult to understand I can provide a doc for you

wow, this is great.. very helpful. I wish I had more computers, I only have 4, 3 can't be used for servers, or firewalls- they are peoples PC's :cry:

This is going to come in handy, I'm taking a 'mentor' class at an ISP that I work for, on IP and subnetting.

Sorry, I think you are wrong. 192.168.1.1 is a class C address, so the smaller subnet mask possible is /24. It could be bigger (/24, /25, ... , /32) but cannot be smaller than the defaul subnet mask.