Post by Urbz on Dec 8, 2006 14:40:20 GMT -5
www.certcities.com/forums/forum_posts.asp?tid=3324&pn=1&get=last
The 256 rule is a nice subnetting rule.
The 256 rule is simple, you subtract the subnet mask from 256 to get the block size. For example, 255.255.255.240 has a block size of 256-240, or 16. Meaning the first subnet after the zero subnet is 16, second subnet is 32, first subnets broadcast is 31, host ranges 17~30. That’s it, done.
While the 256 rule is simple to do, sometimes it can get tricky. How many hosts can you get on the network this host is on: 172.16.56.34/23. Well, first step, you need to convert /23 into a subnet mask. If you have the chart, this is simple, just find /23. Without the chart, you have to count. 8,16 and 24 are the “even” subnet masks, 255.0.0.0, 255.255.0.0, 255.255.255.0. Since we know 23 is more then 16, and less then 24, we know it’s 255.255.?.0 mask. What you have to do, is know the valid subnet mask numbers by heart, or be able to figure them out: 0,128,192,224,240,248,252,254,255. /8,/16 and /24 all give you a 0 subnet mask (and 255 on the one before). Add one, /9,/17,/25, you get 128 mask. and just keep going on your counting.
Since we know /24 is 255.255.255.0, you can take a short cut, and use one lower mask. /23 is 255.255.254.0. Now we take the 256 - 254, and we have a block size of 2. The first network is 172.16.0.0 and the second one is 172.16.2.0, up to 172.16.56.0, where our host is on.
You may be thinking, well, block size is 2, subtract the network address and the broadcast address, 2-2 = 0, so something is invalid. Well, it would be, if it wasn’t for the 8 host addresses to the left of it. So the address of the next network is 172.16.58.0, so take one step back, the broadcast address of our 56 network is 172.16.57.255, not 172.16.57.0 like you may have been thinking. Valid host ranges on out network is 172.16.56.1~172.16.57.254. Rather then counting them, you know that there’s 2 in the block size of the c part of the address, 56 and 57, and there’s 256 addresses in each of them, so 256*2 = 512, - the network and broadcast, 510 host addresses. Like I said, anything larger then 510 you may need scrap paper, but any masks of /24 or more, this 256 rule is great.
Edited by Wildcat_Dude on 05 May 2006 at 9:22am
The 256 rule is a nice subnetting rule.
The 256 rule is simple, you subtract the subnet mask from 256 to get the block size. For example, 255.255.255.240 has a block size of 256-240, or 16. Meaning the first subnet after the zero subnet is 16, second subnet is 32, first subnets broadcast is 31, host ranges 17~30. That’s it, done.
While the 256 rule is simple to do, sometimes it can get tricky. How many hosts can you get on the network this host is on: 172.16.56.34/23. Well, first step, you need to convert /23 into a subnet mask. If you have the chart, this is simple, just find /23. Without the chart, you have to count. 8,16 and 24 are the “even” subnet masks, 255.0.0.0, 255.255.0.0, 255.255.255.0. Since we know 23 is more then 16, and less then 24, we know it’s 255.255.?.0 mask. What you have to do, is know the valid subnet mask numbers by heart, or be able to figure them out: 0,128,192,224,240,248,252,254,255. /8,/16 and /24 all give you a 0 subnet mask (and 255 on the one before). Add one, /9,/17,/25, you get 128 mask. and just keep going on your counting.
Since we know /24 is 255.255.255.0, you can take a short cut, and use one lower mask. /23 is 255.255.254.0. Now we take the 256 - 254, and we have a block size of 2. The first network is 172.16.0.0 and the second one is 172.16.2.0, up to 172.16.56.0, where our host is on.
You may be thinking, well, block size is 2, subtract the network address and the broadcast address, 2-2 = 0, so something is invalid. Well, it would be, if it wasn’t for the 8 host addresses to the left of it. So the address of the next network is 172.16.58.0, so take one step back, the broadcast address of our 56 network is 172.16.57.255, not 172.16.57.0 like you may have been thinking. Valid host ranges on out network is 172.16.56.1~172.16.57.254. Rather then counting them, you know that there’s 2 in the block size of the c part of the address, 56 and 57, and there’s 256 addresses in each of them, so 256*2 = 512, - the network and broadcast, 510 host addresses. Like I said, anything larger then 510 you may need scrap paper, but any masks of /24 or more, this 256 rule is great.
Edited by Wildcat_Dude on 05 May 2006 at 9:22am