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CompTIA Network+ Certification Study Guide part 34

CompTIA’s Network+ certification Study Guide part 34 is a globally-recognized, vendor neutral exam that has helped over 235,000 IT professionals reach further and higher in their careers. The 2009 Network+ exam (N10-004) is a major update with more focus on security and wireless aspects of networking. Our new study guide has been updated accordingly with focus on network, systems, and WAN security and complete coverage of today’s wireless networking standards. | 316 CHAPTER 7 TCP IP and Routing Creating the Subnet Mask We ve determined our subnets and now we need to create a subnet mask that will work with each subnet ID we created. Recall that we use bitwise ANDing to compare the bits of the IP address and the subnet mask. The result of the comparison is the network ID. Using Table 6.16 we know that we need to set to 1 any bits used for the network ID portion of the IP address. In this case the subnet mask would be set to 11111111.11111111.11100 000.000000000. Notice that we have set the left-most 19 bits to 1. Thus our subnet masks can be written in dotted decimal notation as 255.255.224.0. Let s compare this subnet mask to a sample IP address from within our subnetted addresses to see how this works. 146.64.193.14 IP address 10010001.01000000.11000001.00001110 255.255.224.0 subnet mask 11111111.11111111.11100000.00000000 Result of bitwise ANDing 10010001.01000000.11000000.00000000 Underlying network ID 146.64.192.0 EXERCISE 7.3 Defining Subnet Masks In this exercise we ll practice defining subnets and subnet masks. Use the following scenario Your brand new start-up company has been assigned a Class C address. You have only six computers one router and three printers attached to your network. You d like to subnet your network before your company s planned expansion and you ll need a maximum of six to seven networks in the future. 1. How many host address bits will you need to take from the host address space to create seven subnets To solve this problem we need to think in terms of the bit value of the binary bits in an octet. Which bit values when added together equal 7 The answer is the right-most three bits or 00000111. This tells us we need three bits from the host address space to add to the network address space. However it s important to remember that we don t use the right-most bits. This may be confusing but we used the bit Understanding Subnet Masking 317 values simply to determine how many bits we ll need. We