How to get subnet mask from an ipv4 address? (college work)

Theminecraftaddict555 · October 22, 2016

1 minute ago, leadeater said:

Subnet mask bits start from the left and move to the right sequentially, a /17 would mean there are 17 bits masked (1) from left to right with the remaining unmasked (0). To figure out the decimal number for each octet you use binary math conversion, there are 8 bits and is base 2 math and you calculate from right to left.

128 64 32 16 8 4 2 1 (all 8 bits, sum total 255)

So for your /17 question the last bit in the 3rd octet must be a 1 so the decimal of 10000000 is 128, so 255.255.128.0.

Here are some previous threads about subnet masks you might find helpful:

https://linustechtips.com/main/topic/599858-cidr-and-number/#comment-7808088

https://linustechtips.com/main/topic/611110-subnetting/#comment-7909923

https://linustechtips.com/main/topic/668967-understanding-the-jist-between-cidr-and-vlsm/#comment-8629688

Got any more questions or want examples ask away, or even questions for you to solve.

Thanks!

BSpendlove · October 22, 2016

I am always up for a private chat if you need any more help with this kind of stuff, kind of topic I love the most now days... I have a few blogs on understanding binary to decimal conversion and vice versa (but many online... I just wrote one to help a friend understand it) (edit: I see leadeater had also said if you have any more questions, trust him if you want a depth understanding )

BSpendlove · October 22, 2016

I also loved people giving me examples in class to work out, so if you now understand from all the previous posts that everyone had kindly put here are some exercises:

What is the subnet mask of the following? (Do them all and double check with some tool online)

11111111.11100000.00000000.00000000

11111111.11111111.11100000.00000000

11111111.11111111.11111110.00000000

11111000.00000000.00000000.00000000

11111111.11111111.11111111.11111000

Try to convert the following into binary:

255.255.192.0

255.224.0.0

255.255.255.252

240.0.0.0

255.255.255.248

While you are on a role, after examples like this working out a wildcard was extremely easy since I was in the zone (danger zone, da.. daa.. da.. dun dun)

255.255.255.0 wildmask is 0.0.0.255 (its somewhat looks like the exact opposite doesn't it? (I felt like it was))

255.255.192.0 wildmask is 0.0.63.255

Don't worry if you don't know what it is used for etc... Just work it out maybe when you are doing subnet mask practise and it will come in handy in the future....

11111111.11111111.11111111.00000000 = 255.255.255.0

00000000.00000000.00000000.11111111 = 0.0.0.255

11111111.11111111.11100000.00000000 = 255.255.224.0

00000000.00000000.00011111.11111111 = 0.0.31.255

Now just a little tip if you are interested.... You don't have to be a genius to sit down and look at something like:

00000000.00000000.00111111.11111111 and in under 3 seconds tell me that in decimal... You really don't..... Just look at 00111111

Just remember the first binary value to the left of the first 1 (00111111) which is.... 64?..... Well matey, I shit you not... Look below and see a pattern which you should understand (but I'll explain after)

Examples:

32

00(0)11111 = 31 (16 + 8 + 4 + 2 + 1?)

128

(0)1111111 = 127 (64 + 32 + 16 + 8 + 4 + 2 + 1?)

64

0(0)111111 = 63 (32 + 16 + 8 + 4 + 2 + 1?)

Maybe notice the pattern: take the 0 to the left of the following 1's... Remember the decimal value of that single positional bit in an octet... and maybe... -1 to get your answer?

P.S hopefully I didn't make a mistake or confuse it too much....

Clyne · October 22, 2016

10 hours ago, BSpendlove said:

I also loved people giving me examples in class to work out, so if you now understand from all the previous posts that everyone had kindly put here are some exercises:

What is the subnet mask of the following? (Do them all and double check with some tool online)

11111111.11100000.00000000.00000000

11111111.11111111.11100000.00000000

11111111.11111111.11111110.00000000

11111000.00000000.00000000.00000000

11111111.11111111.11111111.11111000

Try to convert the following into binary:

255.255.192.0

255.224.0.0

255.255.255.252

240.0.0.0

255.255.255.248

While you are on a role, after examples like this working out a wildcard was extremely easy since I was in the zone (danger zone, da.. daa.. da.. dun dun)

255.255.255.0 wildmask is 0.0.0.255 (its somewhat looks like the exact opposite doesn't it? (I felt like it was))

255.255.192.0 wildmask is 0.0.63.255

Don't worry if you don't know what it is used for etc... Just work it out maybe when you are doing subnet mask practise and it will come in handy in the future....

11111111.11111111.11111111.00000000 = 255.255.255.0

00000000.00000000.00000000.11111111 = 0.0.0.255

11111111.11111111.11100000.00000000 = 255.255.224.0

00000000.00000000.00011111.11111111 = 0.0.31.255

Now just a little tip if you are interested.... You don't have to be a genius to sit down and look at something like:

00000000.00000000.00111111.11111111 and in under 3 seconds tell me that in decimal... You really don't..... Just look at 00111111

Just remember the first binary value to the left of the first 1 (00111111) which is.... 64?..... Well matey, I shit you not... Look below and see a pattern which you should understand (but I'll explain after)

Examples:

32

00(0)11111 = 31 (16 + 8 + 4 + 2 + 1?)

128

(0)1111111 = 127 (64 + 32 + 16 + 8 + 4 + 2 + 1?)

64

0(0)111111 = 63 (32 + 16 + 8 + 4 + 2 + 1?)

Maybe notice the pattern: take the 0 to the left of the following 1's... Remember the decimal value of that single positional bit in an octet... and maybe... -1 to get your answer?

P.S hopefully I didn't make a mistake or confuse it too much....