# Understanding the Configuration Register Bits

Hi

I am reading Todd Lammle for CCNA

I got stuck when I started understanding the configuration register bits

as we know configuration register is 16 bits long, the default configuration on Cisco router is 0x2102 which means bits 13,8 and 1 are onn

as per the table below each set is of 4 bits is read in binary with a value of 8,4,2,1

 Config Register # Bit Number Binary 2 1 0 2 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0    0    1    0 0    0   0 1 0 0 0 0 0 0 1 0

My question is, where this 8,4,2,1 came from ?

• ###### 1. Re: Understanding the Configuration Register Bits

The configuration register is 16 bits.

Hexadecimal represents 4 bits as a single character.

For each hexadecimal character, the 4 bits have the weighting, (from left to right) of 8  4  2 1 (just like the lower 4 bits of an IP address).  They represent powers of 2. Keith

• ###### 2. Re: Understanding the Configuration Register Bits

Hexadecimal is a lot confusing at times unless you understand the basic behind it. If you look at Keith scale there are 4 bits in each section. No matter what the total number is only 15 count can be reach on it. If you look at it like this and keep it simple it will make sence to you in a bit.

Take the first 4 of the IP address bit they are 1,2,4 and 8 add them all together and that will be a max number of 15. That is what they did for the hexadecimal setup.

0-9 are written as number and 10-15 are written as letters. So if you get an letter "F" the total would be 15 "E" is 14 and so on.

For some good books on this, try some DIGITAL electrical books, they go into it a lot and might help you out more also.

• ###### 3. Re: Understanding the Configuration Register Bits

Hello My Techie Friend,

As  you may know that a hexa value is defined in a group of 2 nibbles and it is a base16 solution.

So whenever we talk about a hex value we can break it in nibbles

(128 64 32 16 8 4 2 1 ) values and that's it .... in config register value .. only the first part of nibble is used.

hope it gives you a way .....

--

Parvesh