What is Bit Boundary?

CCNA, CCDA, CCNP, CCDP, CCIP, CCVP and CCIE
Guest

What is Bit Boundary?

Post by Guest » Tue Jun 12, 2007 11:06 am

Hi  Can anybody please explain the concept of Bit boundary and where it is applied in networking? I need a clear explanation for this because I have done an elaborate search in internet but not found to my level so that I can understand  Thanks & Regards Swaminathan

Guest

Re:What is Bit Boundary?

Post by Guest » Tue Jun 12, 2007 12:07 pm

Swaminathan The bit boundary is the bit position in an IP address that marks the dividing point between bit values that are always the same and bit values that will vary. For example if we have a set of addresses with mask of 255.255.255.0 then the bit boundary is at bit 24. Up to bit 24 the bit values will always be the same and beyond bit 24 the bit values may vary. For a slightly more advanced example of bit boundary let us assume that I am preparing the design for a network. The organization has a class B network (perhaps it is 172.20.0.0). In the design I know that the organization has 12 regions and I want to allocate subnets by regions. So the bit boundary would be at bit 20 (16 bits for the class B network and 4 bits to accommodate 12 regions). So the addressing plan would look something like this:region 1: 172.20.0000xxxx.0region 2: 172.20.0001xxxx.0region 3: 172.20.0010xxxx.0region 4: 172.20.0011xxxx.0region 5: 172.20.0100xxxx.0region 6: 172.20.0101xxxx.0...region 12: 172.20.101xxxx.0 So for the subnets of any region the first 20 bits (to the bit boundary) are fixed and the remaining 12 bits are variable. HTH Rick

Guest

Re:What is Bit Boundary?

Post by Guest » Tue Jun 12, 2007 12:57 pm

Hi Richard Burts  Thankyou very much for your reply and now I understood the real meaning of the bit boundary with your very good example and explanation.I appreciate your help regarding this and moreover I found the text as below from  the CCNA book written by Richard Deal :     I needed the explanation for  what the red highlighted statement below mean please?Summarization and Powers of 2When summarizing, however, remember that you can summarize routes only ona bit boundary (power of 2) or a multiple of a power-of-2 boundary. The trick tosummarization is to look at your subnet mask options: 0, 128, 192, 224, 240, 248,252, 254, and 255. Each of these masks covers a range of numbers, as is shown inTable 8-3. For instance, suppose you have a set of Class C subnets: 192.168.1.0/30and 192.168.1.4/30. These networks contain a total of eight addresses and start ona power-of-2 boundary: 0. Therefore, you could summarize these as 192.168.1.0/29,which encompasses addresses from 192.168.1.0 through 192.168.1.7.Let’s take a look at another example. Say you have a set of Class C subnets:192.168.1.64/26 and 192.168.1.128/26. Each of these networks has 64 addresses,for a total of 128 addresses. A mask value that accommodates 128 addresses ina Class C network is 255.255.255.128 (25 bits). However, this subnet mask posesa problem, since the bit value must be a power of 2 and start on a power-of-2 networkboundary. With a 25-bit mask, there are only two network numbers: 192.168.1.0/25and 192.168.1.128/25. The address 192.168.1.64/26 falls under the first networknumber, and the 192.168.1.128/26 falls under the second one—so even thoughthe two networks are contiguous, they can’t be summarized with a 25-bit mask(255.255.255.128). You could use a 24-bit mask (255.255.255.0); however, thisincludes a total of 256 addresses, not just the 128 addresses in question.Summarization DifficultiesThe first two summarization examples were pretty simple. Let’s look at a morecomplicated example to illustrate how difficult summarization can be if youdon’t lay out your addressing correctly in your network. In the network shownin Figure 8-11, Router A needs to summarize routes to which it and Routers B,C, and D are connected, realizing that other networks reside to the left of RouterA. The goal is to have Router A advertise the least number of routes to routersto the networking cloud on the left. Remember that Router A should createsummarizations only for the routes that it is connected to or that are behind it(Routers B, C, and D). Also remember that these summarizations should eitherbe a power of 2 or start on a power-of-2 networking boundary.In this example, the first thing you want to do is put the routes that Router Aknows about (those to its right) in numerical order: 192.168.5.64/28, 192.168.5.80/28,192.168.5.96/28, 192.168.5.112/28, 192.168.5.192/28, and 192.168.5.208/28. Note thatother subnets of 192.168.5.0 reside to the left of Router A that should not be includedin the summarization. In this example, subnets 64, 80, 96, and 112 are contiguous,and if you use a 26-bit summarization mask, this would accommodate addresses from64 through 127. These addresses are contiguous, and the summarization mask startson a power-of-2 network boundary (address 64). To summarize subnets 192 and 208,you would need a 27-bit mask (255.255.255.224), which would include a block of32 addresses: from 192 through 223.Router A can advertise the following summarized routes to the left networkcloud:â–  192.168.5.64/26 This covers addresses 64–127, which are to the right ofRouter A.â–  192.168.5.192/27 This covers addresses 192–223, which are also to theright of Router A.With thanks!S.Swaminathan

Guest

Re:What is Bit Boundary?

Post by Guest » Tue Jun 12, 2007 1:11 pm

S.Swaminathan Let me start my explanation by restating my working definition of bit boundary:The bit boundary is the bit position in an IP address that marks the  dividing point between bit values that are always the same and bit  values that will vary.I believe that this is essentially what he means when he says: power-of-2 network boundaryit is referring to the concept of which bits stay the same when you look at some subnets and what bits vary.His discussion is looking at 2 subnets and determining whether they can successfully be summarized. This particular example is using 192.168.1.64/26 and 192.168.1.128/26 and whether these subnets could be summarized. To see the logic clearly it may be helpful to see the third and fourth octets in binary:1.64  is  00000001.010000001.128 is 00000001.1000000So the bit boundary here is at bit 24 since the bits start to vary at bit 25. Since the bit boundary is at bit 24 it is not successful to attempt to summarize those 2 subnets with a /25 mask. HTH Rick

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