172.16.1.2/20 HOW TO DETERMINE DEFAULT, SUBNET, HOST. PLEASE SHOW ALL THE STEPS

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An IP address is composed of 32 bits. These 32 bits are divided into 4 octets of 8 bits each. You may have seen an IP address represented like this: 172.68.15.24. We must remember, however, that the computer understands this number only in binary, so we must often deal with them in binary. Many people are intimidated by this initially, but soon find that it is not difficult. If you do not allow yourself to be flustered, you can master this topic. IP addresses are assigned to orginazations in blocks. Each block belongs to one of three classes: class A, class B, or class C. You can tell what class an IP address is by the value in its first octet. Class A 1-126 Class B 128-191 Class C 192 --> An IP address consists of two fields. The first field identifies the network, and the second field identifies the node on the network. Which bits of the address are in the network field and which bits are in the host field is determined by the subnet mask. 3.3 Simple Subnetting The simpliest way to subnet is to take the octet in the subnet mask that covers the first unassigned octet in the IP address block, and make all its bits high. Remember, a high bit, a 1, in the subnet mask indicates that that corresponding bit in the IP address is part of the network field. So, if you have a class B network 172.160.0.0, with the subnet mask 255.255.0.0, you have one network with 65, 534 possible addresses. If you take that subnet mask and make all the bits in the third octet high 128 64 32 16 8 4 2 1 --- -- -- -- - - - - 1 1 1 1 1 1 1 1 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255 you get the subnet mask 255.255.255.0. 172.60. 0. 0 255.255.255.0 Now the third octet of all the addresses on this network are part of the network field instead of the host field. That is one octet, or eight bits, that can be manipulated to create subnets. 2^8=256 possible subnets now on this class B network. One octet is left for the host field. 2^8-2=254 possible host addressed on each subnet. 3.4 Advanced Subnetting That is the simplist way to subnet, but it may not be the most desirable. You might not want 256 subnets on your class B network. Instead, you might use a subnet mask like 255.255.224.0. How many subnets would this give you? The first step is to see how many bits are allocated to the network by this mask. 128 64 32 16 8 4 2 1 --- -- -- -- - - - - 1 1 1 0 0 0 0 0 128 + 64 + 32 + 0 + 0 + 0 + 0 + 0 = 224 3 bits are allocated. 2^3=8 subnets. How many hosts on each subnet? Well, 5 bits from this octet are left for the host field, and 8 bits in the fourth octet, for a total of 13 bits in the host field. 2^13-2=8190 possible hosts on each subnet. The subnet mask is always extended by masking off the next bit in the address, from left to right. Thus, the last octet in the subnet mask will always be one of these: 128, 192, 224, 240, 248, 252, 254 or 255. Given the IP address of a host and the subnet address for the network, you need to be able to calculate which subnet that host is on. To do this we compare the binary representation of the pertinent octet of the subnet mask witht he binary representation of the corresponding octet in the IP address. Example: IP address=172.60.50.2 subnet mask=255.255.224.0 50= 00110010 224=11100000 We perform a logical AND on these two numbers. We will be left with only the bits where there is a one in both octets. 00110010 11100000 -------- 00100000=32 This host is on subnet 172.60.32.0. We also need to be able to find the range of assignable IP addresses on this subnet. To do this, we take the binary that tells us the subnet address, in this case 00100000, and compare it with the subnet mask. 00100000 11100000 The bits convered by the mask we will leave as they are. The rest of the bits we make high. So 00100000 11100000 -------- 0011111=63 The range of assignable IP addresses on the subnet 172.60.32.0 is 172.60.32.1-172.60.63.254. On every network and subnet, two addresses are reserved. At the low end of the range of addresses for the network or subnet, in this case 172.60.32.0, is the address for the network or subnet itself. The address at the high end of the range of addresses, in this case 172.60.63.255, is the broadcast address. Any message sent to the broadcast address will be received by every host on the network.

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