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LinkBack | Thread Tools |
11-11-2005, 01:01 PM
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#1 (permalink) |
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Registered User
Join Date: Feb 2005
Posts: 82
OS: XP Pro
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Subnetting
Studying subnetting and I found an issue.
Question: How many subnets? Subnet mask class C address 255.255.255.224 Binary: 11111111.1111111.11111111.11100000 borrowed 3 bits from 4th octet in host range 2^3-2 = 6 subnets Found this reference/example on many online sites. Here's one: http://home.nyc.rr.com/computertaijutsu/subnet.html Went to Cisco site and printed out "Cisco-IP Addressing and Subnetting for New Users" and its example of subnetting a class C address when borrowing 3 bits from the hosts portion = 8 subnets. http://www.cisco.com/warp/public/701/3.html The formula for finding out how many subnets is 2^n-2 where n = number of "on bits" or 1's. How come Cisco's example says 8 subnets? My IP calculator on my Palm even computes a class C address with 3 bits borrowed and /27 as having 8 subnets. What am I missing here? Andy |
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11-13-2005, 12:36 AM
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#2 (permalink) |
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Retired
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Hmm ...
192.168.0.0/27 ... 192.168.0.0 - 192.168.0.31 192.168.0.32 - 192.168.0.63 192.168.0.64 - 192.168.0.95 192.168.0.96 - 192.168.0.127 192.168.0.128 - 192.168.0.159 192.168.0.160 - 192.168.0.191 192.168.0.192 - 192.168.0.223 192.168.0.224 - 192.168.0.255 Looks like 8 valid networks/subnets to me ...  Assuming Class C: Last Octet .... Networks/Hosts per Network .0 ................ 1 / 254 .128 ............. 2 / 126 .192 ............. 4 / 62 .224 ............. 8 / 30 .240 ............. 16 /14 .248 ............. 32 / 6 .252 ............. 64 / 2 (each network listed has 2 additonal ip's for network and broadcast) 2^n, where n is the number of bits set to 1 in the last octet in the mask to use 1's that is NOT = 255, must equal or exceed the number of desired subnets. If you want 9 subnets, then your mask must be 2^4 = 16, or .11110000 (2^3 = 8 ... not enough) 2^n-2, where n is the number of bits set to 0 in the ENTIRE mask, is the number of valid hosts per network. If the mask is 255.255.255.240, then you have an ending octect of .11110000. 4 0's, 2^4 = 16 - 2 = 14 hosts per network. |
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