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Subnet masks

post #1 of 6
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hey everyone,
i am studying the Cisco CCNA course at college, and i have reached the part where it discusses subnet masking, and i have to say that it sucks bad

this has been the only chapter in the semister so far that i havent been able to figure out.

i understood TCP/IP and its classes but this is starting to get ridiculous

someone told me that:
255 . 255 . 255 . 0 equals to
11111111.11111111.11111111.00000000 (binary)

i understand the above, but what does that have to do with anything?

i was wondering if anyone knew of an easy way to do it, or if they got any tips they'd like to share

thanx
    
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post #2 of 6
thats a bit overkill imo , why would you need to know it in binary :C
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post #3 of 6
Doing it in binary makes it much more visual. 255.255.255.0 is no big deal, but when you have 255.255.255.240, it's easier to do it in binary. If someone gave you the IP address of 192.168.168.168. and the subnet mask of 255.255.255.240, you can use binary to figure out the network address, broadcast, address, usable IP's, etc.

First of all, since the subnet mask starts with 255.255.255, you know that the 192.168.168 is all part of the network portion. All you need to be concerned with is the 168, and the 240.

10101000 = 168
11110000 = 240

1111 is the network portion, and 0000 is the host portion

1010 | 1000
1111 | 0000

If you take 1010 and add 4 zeros to the end, you get 10100000, or 160 in decimal. Similarly, add 4 zeros to the beginning 1000, and you get 00001000, or 8 in decimal.

So, now you know that 192.168.168.168 is part of the 192.168.168.160 network. If you convert 1111 do decimal, you get 15. Add the zero, and there are 16 possible IP addresses. Since 192.168.168.160 is the network address, and 192.168.168.175 is the broadcast address, then you have 14 usable ip addresses.
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post #4 of 6
Quote:
Originally Posted by Crimson_Blade
someone told me that:
255 . 255 . 255 . 0 equals to
11111111.11111111.11111111.00000000 (binary)

i understand the above, but what does that have to do with anything?
I just thought of something to add. All of the 1's represent the network portion of the subnet mask. The first zero represents where the host portion of the mask starts. Any 1's after the first zero don't mean anything, which is why you will never see a subnet mask like 255.255.255.241.

255.255.255.240 looks like:

11111111.11111111.11111111.11110000

So, if you have the IP address 192.168.168.168, convert it to binary, and compare it to the subnet mask:

11000000.10101000.10101000.10101000
11111111.11111111.11111111.11110000
<--------network portion--------->^^^host

See how visual that is? If you look at the last octet, 1010 is part of the network portion, and 1000 is part of the host portion. I explained more detail about this in my above post.

Subnetting can be really difficult to understand. I had a real hard time with it for months and months until the light switch clicked in my head one day. Once you get it, you will stop writing subnet masks as 255.255.255.240, and start writing: 192.168.168.160/28. The /28 represents the number of bits in the subnet mask. If you add up all the 1's in 255.255.255.240, you will get 28.

That's it for class today. Have a nice day.
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post #5 of 6
took the words right out of my mouth blink. yes subnetting sucks so bad. but the fundimentals are pretty priceless when you start configuring ios's on the equipment. all i can say is master your binary conversion.
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post #6 of 6
Quote:
Originally Posted by Tezzanator
thats a bit overkill imo , why would you need to know it in binary :C
binary is so imperitive to CCNA
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