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Subnetting Shortcuts

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

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Subnetting Shortcuts

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  1. Subnetting Shortcuts Batavia High School Cisco Networking Academy Semester 3 Mr. Mullen

  2. 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

  3. Let’s cut to the chase! Class B IP address example: 145.31.0.0 10010001.00011111.00000000.00000000 Network Bits Host Bits The subnet mask would be: 255.255.0.0 11111111.11111111.00000000.00000000

  4. 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 3rd octet of the Network ID 10010001.00011111.00000000.00000000 Network Bits Host Bits Subnet Bits

  5. What is the result? • The new subnet mask must reveal the borrowed bits: 11111111.11111111.11111111.00000000 255.255.255.0 Borrowing rule: 2N = total subnets needed N = #bits borrowed = 8 28 = 256 total subnets

  6. What about the # of hosts? 11111111.11111111.1111111.00000000 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: 2N = total hosts possible N = #bits remaining = 8 28 = 256 total hosts possible/subnet Subnet Bits Host Bits

  7. 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 0 145.31.0.0 145.31.0.1 - 145.31.0.254 145.31.0.255 1 145.31.1.0 145.31.1.1 – 145.31.1.254 145.31.1.255 2 145.31.2.0 145.31.2.1 – 145.31.2.254 145.31.2.255 3 145.31.3.0 145.31.3.1 – 145.31.3.254 145.31.3.255 See any pattern here? What would SN 212 look like? 212 145.31.212.0 145.31.212.1 – 145.31.212.254 145.31.212.255

  8. 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 = 29 = 512 Hosts = 2(16-9) =27 = 128 SN host range #s: .0 - .127 and .128 - 255

  9. And today’s winning SN #’s are…512 Total Subnets with 128 Possible Hosts\SN Note: when the SN# is even, it is just 2x’s the value of the 3rd octet Note: when the SN# is even, the value of the 4th octet begins at 0 and when SN# is odd, the value of the 4th octet begins at 128

  10. How about this one… Let’s look at another example using a Class C address: IP Address = 207.142.25.0 SN Mask = 255.255.255.0 Need to create 14 subnets, will have to borrow 4 host bits (24 = 16). 11111111.11111111.11111111.11110000 New SN Mask = 255.255.255.240

  11. Let’s have some fun with the #’s 16 remember that # # of Subnets = 2n = 24 = 16 # of Hosts = 24 = 16 Note: that the 4th octet value for the SNA is 16x’s the value SN# Note: vertically all the blue numbers increase by a value of 16

  12. So the next time you are getting frustrated with subnetting…. Play around and have some fun with the numbers!

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