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I'm a student of electronics and get lessons in networking as well. But on one of my tests I got the question:

What subnet mask is indicated by the network address 172.16.4.8/18?

I know the answer is 255.255.192.0 but I don't understand why the answer must be 255.255.192.0 and how to calculate the subnet mask? Also what does the /18 mean?

marked as duplicate by YLearn May 30 '14 at 22:30

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You must read a subnet mask in binary. Since your IP address ends with /18 it means the first 18 bits are high and the others are low. Getting a binary code of 11111111.11111111.11000000.0000. Put it back to decimals and you'll get 255.255.192.0

  • So in other words when I have an IP address like 172.168.4.6/24 the subnet mask will be 255.255.255.0? And 172.168.4.8/32 will be 255.255.255.255? – user5913 May 30 '14 at 22:30
  • That is correct. – user43384 May 30 '14 at 22:31
  • Ok thanks man! That cleared things up for me! Thanks for the fast reply! – user5913 May 30 '14 at 22:32

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