# Subneting/Borrowing bits question

Registered Users Posts: 1 ■□□□□□□□□□
Hello all.

I have an excersize for university which I can't finish and bothers me a lot. If someone can help, I would really appreciate it.

The question is - In a network with an IP Adress 10.0.0.0/8, how many bits you need to borrow, to create a 100 subnets? What will be the network prefix for the subnets?

• Member Posts: 154 ■■■□□□□□□□
100 subnets you need 7 bits. So 10.0.0.0/15 is the address and prefix.

Disclaimer: I suck at subnetting, trust the next person who comes along instead lol =P
• CCNP R/S, CCNA R/S, CCNA SEC, A+, NET+, SEC+, PROJECT+, LINUX+, LPIC-1, Palo ACE Member Posts: 117 ■■■□□□□□□□
https://www.subnetting.net/ This site really helped me when I was first learning subnetting.

To answer your question, Nik was correct. You're starting out with 10.0.0.0/8. That's one subnet. You need 100. Every bit you "borrow" for more subnets gives you 2^n subnets. So if you borrow 1 bit, now you have 2 subnets (2^1 = 2). If you borrow 5 bits, you have 32 subnets (2^5 = 32).

You need 100 subnets, so start counting in powers of 2 until you reach 100+, and that's how many bits you need to borrow. 2, 4, 8, 16, 32, 64, 128. That's 2^7 = 128. So you need 7 bits. Add that 7 to your original 8, and now you have 10.0.0.0 /15 as your network.

As for the prefix in each subnet, you need to convert that /15 into your mask. A /15 would be 255.254.0.0. So your first network is 10.0.0.0.
Now, they way I figure out what my different network IDs are, you take your mask, 255.254.0.0, and find the first octet that isn't 255. So that's our second octet, .254. Subtract 256 from 254, and you get 2. So your network IDs will increment by 2 in the second octet.

So, your network ID's will look like:
10.0.0.0
10.2.0.0
10.4.0.0
10.6.0.0
......
10.252.0.0
10.254.0.0

And at this point, you've used up your 128 created subnets for the 10.0.0.0 network since you can't have 10.256.0.0.
CCIE RS - Written (Goal: July 2019) [ ] Lab [ ]
• https://www.udemy.com/ip-subnetting/

this is a good course for subnetting on udemy which is free.