![]() ![]() What summary address should you use? Well there's 10 subnets. You need to summarize the following range: An example below shows you how this could happen: Experience with supernetting questions will help you to identify this. Sometimes you may not be able to get all of the addresses into a supernet without wasting addresses. There are, however, a few pitfalls with supernetting/summarization. 2 to the power of 2 gives us 4 so default mask of 24, minus 2, gives us /22. Quick, eh? This is how it works in my head, "mmmm, 16 addresses, 2 to the 4 is 16, mask is 16, minus 4 is 12, so summary address must be first address with /12 mask." Add this mask to the first address in the range - 172.16.0.0/12 in this example In this example our default mask is 16 so the mask after subtracting 4 is /12.Ĥ. Subtract the figure from step 2 from the default mask of our address range. What power of 2 equals our range? 16 subnets = 2 4 so the answer is four.ģ. How many subnets are in the range? The RFC1918 Class B range is 16 subnets.Ģ. But can we be quicker? Yes we can is the good news, after all, you don't want to eat up time in the exam by writing out addresses in binary. We therefore start at our first address 172.16.0.0 and append our mask so the summary address is 172.16.0.0/12. If we look at it we see that the first 12 bits for each address is the same so that gives us our mask in slash notation. ![]() From this list, count from the left how many bits are the same in each address. Write out the binary equivalent of the address up to and including the changing octet. For example, the Class B RFC1918 range, the IP address first changes in the second octet (i.e. Starting from the left of the IP address, identify the first octet that has a change of address in it. This is what you really want to know isn't it? Well it's dead simple.ġ. Thus when the link goes up or down, you don't have a flurry of traffic announcing the state change.* The less obvious benefit is that summarization means you're tracking whether or not you're connected to some subnets of a summary, not the up/down state of every link. This in turn means that memory requirements are reduced. Quite clearly, if we have just one address instead of lots of individual addresses then the routing table is going to be smaller. You see what has happened there? We've taken a range of addresses and squashed it into one advertisement. The most well-known summarization/supernet is the RFC1918 Class B Range. The process of taking a range of IP addresses and advertising them in one address block. I also get this with "neighbor" versus "neighbour".Īnyway, I digress. ![]() Some people get really confused when it comes to route summarization, probably more so with the different names given to it:Īctually, I get more confused typing out this thing as being English I want to replace the z in "summarization" with an s, but as it's Cisco I best keep the z in, lol. ![]()
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