Subnetting Shortcuts Batavia High School Cisco Networking Academy Semester 3 Mr. Mullen
If you’re going to cut…..make it a short-cut in the way you get something done, not a class! Subnetting shortcuts are a way to: –Determine subnet addresses –Determine a specific host address within a specific subnet. –Take the computational confusion out of determining a subnet address range
Let’s cut to the chase! Class B IP address example: Network Bits Host Bits The subnet mask would be:
Remember just a few things.. We’ll start small and easy, suppose we want to create a total of 256 SN’s. Starting from the left, we need to borrow the 8 most significant host bits or the entire 3 rd octet of the Network ID Network Bits Host Bits Subnet Bits
What is the result? The new subnet mask must reveal the borrowed bits: Borrowing rule: 2 N = total subnets needed N = #bits borrowed = = 256 total subnets
What about the # of hosts? Remember there are 16 total host bits in a Class B, we have borrowed 8 for subnetting. That leaves us with 8 host bits. Using the # of hosts rule: 2 N = total hosts possible N = #bits remaining = = 256 total hosts possible/subnet Host Bits Subnet Bits
Cursive review, so far, Right? Let’s look at the first few subnets. Remember, the first subnet is not useable, but lets list it anyways. SN #SNA Useable Range Broadcast – – – See any pattern here? What would SN 212 look like? –
So what’s the big deal? Nothing really! Let’s look at another example for the same address, but lets create 512 total subnets this time. Subnets = 2 9 = 512 Hosts = 2 (16-9) = 2 7 = 128 SN host range #s: and
SN #SNAUseable RangeBroadcast – – – – – See any pattern here? What would SN 212 & 213 look like? SN 450 & 451? – – Note: when the SN# is even, it is just 2x’s the value of the 3 rd octet Note: when the SN# is even, the value of the 4 th octet begins at 0 and when SN# is odd, the value of the 4 th octet begins at 128 And today’s winning SN #’s are… 512 Total Subnets with 128 Possible Hosts\SN
How about this one… Let’s look at another example using a Class C address: IP Address = SN Mask = Need to create 14 subnets, will have to borrow 4 host bits (2 4 = 16) New SN Mask =
Let’s have some fun with the #’s # of Subnets = 2 n = 2 4 = 16 # of Hosts = 2 4 = 16 SN #SNAUseable RangeBroadcast See any pattern here? What would SN 8 & 9 look like? SN 14 & 15? – Note: that the 4 th octet value for the SNA is 16x’s the value SN# Note: vertically all the blue numbers increase by a value of remember that #
So the next time you are getting frustrated with subnetting….