Subnetting

Subnet-Mask2

Part 5:

Determining the Subnet number of a given IP Address

An example of a “determine the subnet number” question:

“What subnet is the address 200.17.49.200 /23 a member of?” or “On what subnet can the address 200.17.49.200 /23 be found?”

Subnet-Mask

This is one of those types of questions that often trips up CCNA candidates. It is because many don’t understand the Boolean AND operation, which is the only way you can properly answer this question. This segment will review the Boolean AND operation and show you how to use it in order to solve this question. As with anything else within this subject matter, once you are used to using the Boolean AND operation, everything else (of similar subject) tends to be easier.

The Boolean AND is, simply put, a bit-by-bit comparison of the IP address and a subnet mask. In this case, the Boolean AND will reveal the subnet upon which this IP address esists.  Your knowledge of binary math will be, and always is, key in you answering this question type as well, since the address and mask must be broken down into binary in order to perform the Boolean AND. 

You must use the skills of “Converting Dotted Decimal To Binary”, to convert the IP address to binary:

128      64       32       16        8         4          2          1

1st Octet: 200                         1          1          0          0          1          0          0          0

2nd Octet: 17             0          0          0          1          0          0          0         1

3rd Octet: 49               0          0          1          1          0          0          0         1

4th Octet: 200                         1          1          0          0          1          0          0         0

The IP address, in binary, is 11001000 00010001 00110001 11001000.  Your knowledge of prefix notation tells you that a subnet mask of /23 is 11111111 11111111 11111110 00000000. (The first 23 bits are ones).  Now that the IP address and subnet mask have been converted to binary, the subnet on which the IP address resides can be found by performing a Boolean AND. Remember, a Boolean AND is simply a bit by-bit comparison of the address and mask.

Bit 1    Bit 2    Bit 3    Bit 4    Bit 5    Bit 6    Bit 7    Bit 8

IP Address      1          1          0          0          1         0         0          0                                       Octet 1

Subnet Mask   1          1          1          1          1         1         1          1                                       Octet 1      

Note that where a bit in the same position is “1” in both the IP address and subnet mask, the Boolean AND result is also “1”. Any other combination results in the Boolean AND resulting in “0”.  And now that we’ve looked at the Boolean AND being run on a single octet, let’s run it on the entire IP address and subnet mask. This is the chart you should use on exam day to answer this question type:

Octet 1                       Octet 2            Octet 3            Octet 4

IP Address                  11001000        00010001        00110001        11001000                      200.17.49.200

Subnet Mask               11111111        11111111        11111110        00000000                      255.255.254.0 (/23)

Boolean AND            11001000        00010001        00110000        00000000                      Result

Subnet-Mask1

Once the Boolean AND result is achieved, it has to be converted into dotted decimal. Using your knowledge of converting binary to dotted decimal, you see that the IP address you were given is found on the 200.17.48.0 /23 subnet.

128      64        32        16        8          4          2          1

First Octet       1          1          0          0         1          0          0          0          200

Second Octet 0          0          0          1          0          0          0          1          17

Third Octet     0          0          1          1          0          0          0          0          48

Fourth Octet   0          0          0          0          0          0          0          0          0

You can now see where the skills you learned in earlier sections come into play in the more complex subnetting questions. When you master the fundamentals of binary math, as you have, you can answer any question Cisco gives you.

See also:  Part 1, Part 2, Part 3, Part 4, Part 5

 

Works Cited

Bryant, C. (2007). The Ultimate CCNA Study Package – ICND 1 And 2: Valid Hosts. In C. Bryant, The Bryant Advantage (p. 6).

Lammle, T. (2007). CCNA Cisco Certified Network Associate Study Guide. Indianapolis: Wiley Publishing, Inc.

Odom, W. (2012). Official Cert Guide ICND1 640-822. Indianapolis, IN: Cisco Press.

Odom, W. (2011). Official Cert Guide ICND2 640-816. Indianapolis, IN: Cisco Press.