31 Days Before Your CCNA Exam

Day 21IPv4 Address SubnettingCCNA 640-802 Exam Topics■Calculate and apply an addressing scheme, including VLSM IP addressing design to a network.■■Determine the appropriate classless addressing scheme using VLSM and summarization tosatisfy addressing requirements in a LAN/WAN environment.Describe the operation and benefits of using private and public IP addressing.Key TopicsBy now, you should be able to subnet very quickly. For example, you should be able to quicklyanswer a question such as: If you are given a /16 network, what subnet mask would you use thatwould maximize the total number of subnets while still providing enough addresses for the largestsubnet with 500 hosts? The answer would be 255.255.254.0 or /23. This would give you 128 subnetswith 510 usable hosts per subnet. You should be able to calculate this information in veryshort order.The CCNA exam promises to contain lots of subnetting and subnetting-related questions. Todaywe focus on this necessary skill as well as designing addressing schemes using variable-lengthsubnet masking (VLSM) and summarization to optimize your network routing traffic. We alsoreview the difference between public and private addressing.IPv4 AddressingAlthough IPv6 is rapidly being deployed on the Internet backbone, and all U.S. government networkswere required to be IPv6-capable by June 30, 2008, migration away from IPv4 will takeyears to complete. Even though U.S. government networks are now IPv6 capable, this does notmean they are actually running IPv6. So IPv4 and your skill in its use is still in demand.Header FormatTo facilitate the routing of packets over a network, the TCP/IP protocol suite uses a 32-bit logicaladdress known as an IP address. This address must be unique for each device in the internetwork.Figure 21-1 shows the layout of the IPv4 header.Note that each IP packet carries this header, which includes a source IP address and destination IPaddress.