How to calculate if a given host is in the same subnet as another
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| Variables: |
IPA |
Address of Host A |
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TMP |
temporary variable |
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NMA |
Netmask of Host A |
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CGU |
Result |
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IPB |
Address of Host B |
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If zero: |
Hosts are in same subnet |
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NMB |
Netmask of Host B |
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If not zero: |
Hosts are not in same subnet |
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| Operations: |
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Host A |
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Host B |
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TMP |
= IPA xor IPB |
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TMP |
= IPA xor IPB |
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CGU |
= TMP and NMA |
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CGU |
= TMP and NMB |
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"xor" and "and" are binary operations that mask one binary number with another |
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Xor: |
Sets all positions to "1" that are different in the two numbers and all positions to "0" that are equal. |
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Example: 0101 xor 0011 = 0110 |
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And: |
Sets all positions to "0" in the two numbers except for those where both are "1" |
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Example: 0101 and 0011 = 0001 |
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| Examples |
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Host A |
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Host B |
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| IPA |
192.168.1.1 (=10000000.10101000.00000001.00000001) |
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IPB |
192.168.1.2 (=10000000.10101000.00000001.00000010) |
| NMA |
255.255.255.0 (=11111111.11111111.11111111.00000000) |
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NMB |
255.255.255.0 (=11111111.11111111.11111111.00000000) |
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TMP |
= IPA xor IPB |
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= 192.168.1.1 xor 192.168.1.2 = 0.0.0.3 (=00000000.00000000.00000000.00000011) |
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CGU |
= TMP and NMA = 0.0.0.3 and 255.255.255.0 = 0.0.0.0 |
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The hosts are in the same subnet |
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| IPA |
192.168.1.1 |
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IPB |
192.168.5.2 |
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| NMA |
255.255.255.0 |
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NMB |
255.255.255.0 |
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TMP |
= IPA xor IPB |
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= 192.168.1.1 xor 192.168.5.2 = 0.0.4.3 |
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CGU |
= TMP and NMA = 0.0.4.3 and 255.255.255.0 = 0.0.4.0 |
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The hosts are not in the same subnet |
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| IPA |
192.168.1.1 |
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IPB |
192.168.5.2 |
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| NMA |
255.255.0.0 |
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NMB |
255.255.255.0 |
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TMP |
= IPA xor IPB |
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TMP |
= IPA xor IPB |
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= 192.168.1.1 xor 192.168.5.2 = 0.0.4.3 |
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= 192.168.1.1 xor 192.168.5.2 = 0.0.4.3 |
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CGU |
= TMP and NMA = 0.0.4.3 and 255.255.0.0 = 0.0.0.0 |
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CGU |
= TMP and NMB = 0.0.4.3 and 255.255.255.0 = 0.0.4.0 |
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For Host A, the hosts are in the same subnet |
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For Host B, the hosts are not in the same subnet |