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What are the subnet masks for the following slash address blocks? Can any one explain why?

Rocky Liu
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2 Answers2

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The subnet mask is a 32 bit number, just like an IP address.

The number after the slash tells you the number of consecutive 1s in the mask. So in the first example, there are eight consecutive ones:

11111111 00000000 00000000 00000000

When you write that in dotted decimal notation, you get 

255.0.0.0

The next example:

11111111 11111110 00000000 00000000

or 255.254.0.0

I'll leave the last one to you ;)

Ron Trunk
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If you found this page and are just looking for a quick lookup table, here it is:

/32 255.255.255.255
/31 255.255.255.254
/30 255.255.255.252
/29 255.255.255.248
/28 255.255.255.240
/27 255.255.255.224
/26 255.255.255.192
/25 255.255.255.128
/24 255.255.255.0
/23 255.255.254.0
/22 255.255.252.0
/21 255.255.248.0
/20 255.255.240.0
/19 255.255.224.0
/18 255.255.192.0
/17 255.255.128.0
/16 255.255.0.0
/15 255.254.0.0
/14 255.252.0.0
/13 255.248.0.0
/12 255.240.0.0
/11 255.224.0.0
/10 255.192.0.0
/9 255.128.0.0
/8 255.0.0.0
/7 254.0.0.0
/6 252.0.0.0
/5 248.0.0.0
/4 240.0.0.0
/3 224.0.0.0
/2 192.0.0.0
/1 128.0.0.0
/0 0.0.0.0
tlund
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