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 3 rd 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: 2 N = total subnets needed N = #bits borrowed = 8 2 8 = 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: 2 N = total hosts possible N = #bits remaining = 8 2 8 = 256 total hosts possible/subnet Host Bits Subnet 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 = 2 9 = 512 Hosts = 2 (16-9) = 2 7 = 128 SN host range #s:.0 -.127 and.128 - 255

9. SN #SNAUseable RangeBroadcast 0145.31.0.0145.31. 0.1 - 145.31.0.126145.31.0.127 1145.31.0.128145.31.0.129 – 145.31.0.254145.31.1.255 2145.31.1.0145.31.1.1 – 145.31.1.126145.31.1.127 3145.31.1.128145.31.1.129 – 145.31.1.254145.31.1.255 4145.31.2.0145.31.2.1 – 145.31.2.126145.31.2.127 5145.31.2.128145.31.2.129 – 145.31.2.254145.31.2.255 See any pattern here? What would SN 212 & 213 look like? SN 450 & 451? 212145.31.106.0145.31.106.1 – 145.31.106.254145.31.106.255 213145.31.106.128145.31.106.129 – 145.31.106.254145.31.106.255 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

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 (2 4 = 16). 11111111.11111111.11111111.11110000 New SN Mask = 255.255.255.240

11. Let’s have some fun with the #’s # of Subnets = 2 n = 2 4 = 16 # of Hosts = 2 4 = 16 SN #SNAUseable RangeBroadcast 0207.142.25.0207.142.25.1 - 207.142.25.14207.142.25.15 1207.142.25.16207.142.25.17 - 207.142.25.30207.142.25.31 2207.142.25.32207.142.25.33 - 207.142.25.46207.142.25.47 3207.142.25.48207.142.25.49 - 207.142.25.62207.142.25.63 See any pattern here? What would SN 8 & 9 look like? SN 14 & 15? 8207.142.25.128207.142.25.129 - 207.142.25.142207.142.25.143 9145.31.106.144145.31.106.145 – 145.31.106.158145.31.106.159 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 16 16 remember that #

12. So the next time you are getting frustrated with subnetting….