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New Member

## How to read the ospf area id on Nexus

Hi All,

When we are going to specify the opsf area id, we can use two different notations:

- A decimal number in the range from 0 to 4294967295

- dotted-decimal notation similar to an IP address, such as A.B.C.D.

Now, let´s assume I want to define one area with id 259.

How can I define it in dotted-decimal notation?

Many thanks.

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

## Re: How to read the ospf area id on Nexus

Hi Fabio,

`Now, let´s assume I want to define one area with id 259.`

This is about integer arithmetics in computers. Notice that a single octet can hold numbers in the range of 0-255. If you want a larger number, you need to use two octets. Here, you can have numbers of the form <0-255>.<0-255> (65536 values in total, 256x256). If you take into account that 255 can be written as 0.255, and 256 is logically 1.0, then 259=256+3=1.3.

When extending it to 4B numbers, this basic principle is retained. Assuming you have an area number N in a linear format, then its dotted decimal format A.B.C.D would be computed as follows:

A = N DIV 2^24

B = (N DIV 2^16) MOD 256

C = (N DIV 2^8) MOD 256

D = N MOD 256

For example, with N = 259:

A = 259 DIV 16777216 = 0

B = (259 DIV 65536) MOD 256 = 0 MOD 256 = 0

C = (259 DIV 256) MOD 256 = 1 MOD 256 = 1

D = 259 MOD 256 = 3

The resulting number is 0.0.1.3. To verify, the linear number should be computed back as A*2^24 + B*2^16 + C*2^8 + D = 0+0+1*256+3 = 259

With N = 942639582:

A = 942639582 DIV 16777216 = 56

B = (942639582 DIV 65536) MOD 256 = 14383 MOD 256 = 47

C = (942639582 DIV 256) MOD 256 = 137

D = 942639582 MOD 256 = 22

The resulting number is 56.47.137.222. To check back: A*2^24 + B*2^16 + C*2^8 + D = 56*2^24 + 47*2^16 + 137*2^8 + 222 = 942639582.

Do you have a Python interpreter handy? It's quite easy to write a script to convert between these two formats.

Best regards,

Peter

2 REPLIES
Cisco Employee

## Re: How to read the ospf area id on Nexus

Hi Fabio,

`Now, let´s assume I want to define one area with id 259.`

This is about integer arithmetics in computers. Notice that a single octet can hold numbers in the range of 0-255. If you want a larger number, you need to use two octets. Here, you can have numbers of the form <0-255>.<0-255> (65536 values in total, 256x256). If you take into account that 255 can be written as 0.255, and 256 is logically 1.0, then 259=256+3=1.3.

When extending it to 4B numbers, this basic principle is retained. Assuming you have an area number N in a linear format, then its dotted decimal format A.B.C.D would be computed as follows:

A = N DIV 2^24

B = (N DIV 2^16) MOD 256

C = (N DIV 2^8) MOD 256

D = N MOD 256

For example, with N = 259:

A = 259 DIV 16777216 = 0

B = (259 DIV 65536) MOD 256 = 0 MOD 256 = 0

C = (259 DIV 256) MOD 256 = 1 MOD 256 = 1

D = 259 MOD 256 = 3

The resulting number is 0.0.1.3. To verify, the linear number should be computed back as A*2^24 + B*2^16 + C*2^8 + D = 0+0+1*256+3 = 259

With N = 942639582:

A = 942639582 DIV 16777216 = 56

B = (942639582 DIV 65536) MOD 256 = 14383 MOD 256 = 47

C = (942639582 DIV 256) MOD 256 = 137

D = 942639582 MOD 256 = 22

The resulting number is 56.47.137.222. To check back: A*2^24 + B*2^16 + C*2^8 + D = 56*2^24 + 47*2^16 + 137*2^8 + 222 = 942639582.

Do you have a Python interpreter handy? It's quite easy to write a script to convert between these two formats.

Best regards,

Peter

New Member

## Re: How to read the ospf area id on Nexus

Hi Peter,

Clear, it make sense.

Thanks!!

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