Given an IP Address and Mask What Are the Assignable IP Addresses in That Network Subnet

Last Updated on Sun, 27 Aug 2023 | Interface Serial

CCNAs deal with the question, "What IP addresses are in this subnet?" on a regular basis. This section describes how to answer this question if you know how to derive the subnet and broadcast addresses. Simply put, the valid IP addresses that are available for assignment in a subnet are those numerically between the subnet number and the broadcast address.

There is little else to be considered. Certainly, no decimal or binary algorithm needs to be considered for such a simple concept, right? Table 5-19 shows some familiar IP addresses and the corresponding IP addresses in the same subnet.

Table 5-19 Assignable Addresses

Subnet Number

Subnet Mask

Broadcast Address

Range of Assignable Addresses

130.4.100.0

255.255.252.0

130.4.103.255

130.4.100.1 to 130.4.103.254

199.1.1.0

255.255.255.224

199.1.1.31

199.1.1.1 to 199.1.1.30

172.100.200.0

255.255.254.0

172.100.201.255

172.100.200.1 to 172.100.201.254

17.9.44.64

255.255.255.192

17.9.44.127

17.9.44.65 to 17.9.44.126

The ranges seem obvious. However, look at the 172.100.200.0 subnet. How many assignable IP addresses are in that subnet, and what are those addresses? Is 172.100.200.255 valid? What about 172.100.201.0? In fact, both are valid IP addresses when using 255.255.254.0 as the

mask. The rule is that the subnet numbers (and also the broadcast addresses) are not usable as an IP address. Otherwise, the addresses are assignable to an interface.

Continue reading here: CIDR Private Addressing and NAT

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Readers' Questions

  • Steven
    How to find the range of ip address assianble?
    2 months ago
    • To find the range of assignable IP addresses, you need to know the network's IP address and subnet mask. Follow these steps: 1. Determine the network's IP address: This is the base IP address of the network. It can be any address within the network's range, but it typically ends with a zero. For example, if the network IP address is 192.168.1.0, that would be the network's IP address. 2. Determine the subnet mask: The subnet mask determines the size of the network and divides the IP address into network and host portions. It is represented in the form of four sets of numbers separated by periods (e.g., 255.255.255.0). 3. Convert the subnet mask to binary: Each number in the subnet mask can be converted to its binary representation. For example, 255 becomes 11111111 in binary. 4. Count the number of binary zeros in the subnet mask: This will indicate the number of available host addresses within the network. 5. Subtract 2 from the total count: This is because the first and last IP addresses in a network are reserved. The first is the network address, and the last is the broadcast address. 6. Calculate the range: Start with the network's IP address and add the number of usable IP addresses (count - 2) to determine the last assignable IP address. For example, if the network IP address is 192.168.1.0 and the subnet mask is 255.255.255.0, here's how to find the range of assignable IP addresses:
    1. Network IP address: 192.168.1.0
    2. Subnet mask: 255.255.255.0
    3. Convert subnet mask to binary: 11111111.11111111.11111111.00000000
    4. Count binary zeros: 8
    5. Subtract 2: 8 - 2 = 6
    6. Range: 192.168.1.1 - 192.168.1.6So, in this example, the range of assignable IP addresses would be from 192.168.1.1 to 192.168.1.6.
    • patrick
      How to identfity how many assignable ip addresses in a given network?
      8 months ago
      • To identify how many assignable IP addresses are in a given network, you will need to know the subnet mask and the IP address range of the network. Using the subnet mask, the IP address range can be divided into different subnets and the total number of assignable IP addresses in each subnet can be calculated.
      • teppo
        What is assignable address?
        1 year ago
        • An assignable address is a type of IP address that is allocated to an individual device within a network. This address is usually assigned by a network administrator and is used to identify a device's location within the network.