Subnetting

ip subnet

Part 6:

Determining Broadcast Addresses And Valid IP Address Ranges For A Given Subnet

No matter the format, you can use your knowledge of binary math to solve this question. You will convert the subnet address into binary, and determine the range of valid addresses as well as the broadcast address at the same time.  Let’s examine how to best answer the “range of valid IP address” question first, and then you’ll see how to quickly determine the broadcast address as well.

address

The question: “What is the range of valid IP addresses for the subnet 210.210.210.0 /25?” As with previous sections, you will use your binary math skills to convert the subnet address and subnet mask into binary. This will allow you to quickly spot the host bits, which are key to answering this question and the broadcast address question. The host bits are those bits set to “0” in the subnet mask.

Octet 1            Octet 2            Octet 3            Octet 4

Subnet Address

210.210.210.0                         11010010        11010010        11010010        00000000

Subnet Mask

255.255.255.128(/25)              11111111        11111111        11111111        10000000

There are three basic rules to remember when determining the subnet address, broadcast address, and range of valid addresses once you’ve identified the host bits as shown above:

1. The address with all 0s for host bits is the subnet address, also referred to as the “all-zeroes” address. This is not a valid host address.

2. The address with all 1s for host bits is the broadcast address, also referred to as the “all-ones” address. This is not a valid host address.

3. All addresses between the all-zeroes and all-ones addresses are valid host addresses, unless the question specifically states otherwise.

You can quickly see that the “all-zeroes” address is 210.210.210.0.  What will the value be if those host bits are set to all 1s? Use your knowledge of binary math to determine this!  The “all-ones” address is 210.210.210.127. If you had trouble making that conversion, review Section Two, “Converting Binary To Decimal”.  This conversion actually answers two different questions. This quick conversion shows you what the range of valid IP addresses is, and also gives you the broadcast, or “all-ones”, address. The second example question, “What is the broadcast address for the subnet 210.210.210.0 /25?”, is answered by using the same method.

Let’s look at another set of examples:

“What is the range of valid IP addresses in the subnet 150.10.64.0 /18?”

“What is the broadcast address of the subnet 150.10.64.0 /18?”

Octet 1            Octet 2            Octet 3            Octet 4

Subnet Address

150.10.64.0                 11010010        00001010        01000000        00000000

Subnet Mask

255.255.192.0 (/18)     11111111        11111111        11000000        00000000

If all the host bits are “zeroes”, the address is 150.10.64.0, the subnet address itself. This is not a valid host address.  If all the host bits are “ones”, the address is 150.10.127.255. That is the broadcast address for this subnet.  All bits between the subnet address and broadcast address are considered valid addresses. This gives you the range 150.10.64.1 – 150.10.127.254. 

Again, the method used to arrive at the range of valid IP addresses is the same as that used to discover the broadcast address of a given subnet.  Let’s take a look at the other question type from the first part of this section:

“Which of the following IP addresses are found on the same subnet as the IP address 210.210.210.130 /25?”

“Which of the following IP addresses are not found on the same subnet as the IP address 210.210.210.130 /25?”

subnet

For some subnetting questions, you’re going to have to determine more than one factor before you can give the correct answer. This question looks simple enough on the surface, but to answer this question type correctly, you must determine two things:

1. On what subnet can this address be found?

2. What is the range of valid IP addresses for this subnet?

In the example, you must first determine the subnet address of the IP address in question, which you learned how to do in Section Six:

Octet 1            Octet 2            Octet 3            Octet 4

IP Address

210.210.210.130                     11010010        11010010        11010010        10000010

Subnet Mask

255.255.255.128 (/25)             11111111        11111111        11111111        10000000

Boolean AND Result              11010010        11010010        11010010        10000000

Converting The Boolean AND Into Dotted Decimal:

128      64        32        16        8          4          2          1          Total

First Octet                   1          1          0          1          0          0          1          0          210

Second Octet              1          1          0          1          0          0          1          0          210

Third Octet                 1          1          0          1          0          0          1          0          210

Fourth Octet               1          0          0          0          0          0          0          0          128

If all the host bits are 0, the all-zeroes address is 210.210.210.128. If all the host bits are 1, the all-ones address is 210.210.210.255. All addresses between these two are valid. You would now look at the different IP addresses presented by the question and then determine which ones fall in the range 210.210.210.129 – 210.210.210.254 (or which ones don’t, if that’s what the question asks for.)

At first, it seems like a lot of work, but as with all other binary math operations, once you practice it, it will become second nature. This question seems longer to solve because it is, since two operations are needed to solve it. Since you’re well-versed in the fundamentals of binary math, this question will present no problems for you.

“Determining Broadcast Addresses” and “Determining Valid IP Address Ranges” Questions What is the valid IP address range for the subnet 222.23.48.64 /26?

Determining The All-Zeroes and All-Ones Subnet Addresses

Octet 1            Octet 2            Octet 3            Octet 4

Subnet Address

222.23.48.64               11011110        00010111        00110000        01000000

Subnet Mask

255.255.255.192         11111111        11111111        11111111        11000000

Identify The Host Bits                                                                            000000

All-Zeroes (Subnet) Address: 222.23.48.64 /26                                                                          

All-Ones (Broadcast) Address: 222.23.48.127 /26                                                                    

Valid IP address range: 222.23.48.65 – 222.23.48.126

EXAMPLE:  What is the valid IP address range for the subnet 140.10.10.0 /23?

Determining The All-Zeroes and All-Ones Subnet Addresses

Octet 1            Octet 2            Octet 3            Octet 4

Subnet Address

140.10.10.0                 10001100        00001010        00001010        00000000

Subnet Mask

255.255.254.0             11111111        11111111        11111110        00000000

All-Zeroes (Subnet) Address: 140.10.10.0 /23                                                                            

All-Ones (Broadcast) Address: 140.10.11.255 /23                                                                    

Valid IP address range: 140.10.10.1 – 140.10.11.254

See also:  Part 1, Part 2, Part 3, Part 4, Part 5, Part 6

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.