IP Address Rulebreakers 3
When first learning about IP addressing and subnetting, it helps to first learn the basics and then practice those basics to build confidence. These IP address rulebreaker exercises help you do just that. Those skills include:
- Recognize dotted-decimal numbers (DDNs) that do not follow the rules for correct IPv4 unicast addresses.
- Apply the pattern to predict the four key facts about a subnet that uses the simplest subnet mask (/24).
- Analyze the addresses shown in a planning diagram to discover:
- Planned addresses with values in the wrong subnet.
- Planned addresses that use reserved values that would not be accepted by the host.
Videos: How to and More Practice
The video on the left teaches what you need to know to do the analysis required in this exercise. There’s also a video with three similar IP address rulebreaker problems (shown on the right.) Enjoy!
The Problems
For each problem, look at the addresses shown beside the router interfaces and PCs. All addresses use the same subnet mask (/24, or 255.255.255.0). Assume that the router IP address is correct. Then consider whether any of the PC IP addresses are incorrect for any reason. The reasons include:
- Invalid values with numbers not allowed in IP addresses
- Valid numbers that are not unicast addresses
- Valid DDN values that are reserved numbers in the subnet (subnet ID, subnet broadcast address)
- Valid DDN values that place the IP address in the wrong subnet
Figure 1: Find the Rulebreakers in this Figure
Answers Below: Spoiler Alert
Identifying the Rulebreakers
Figure 2 shows the rulebreakers for this post with a line through the DDN values. The reasons why are below the figure.
Figure 2: Rulebreakers Identified
To find the rulebreakers, use this set of rules:
All DDN numbers used in IPv4 have a value between 0-255 inclusive in each of the four octets.
Addresses assigned to host interfaces, used as unicast IP addresses, must be from the class A, B, or C range shown in this table:
| First Octet Range | Class |
|---|---|
| 1-126 | A |
| 128-191 | B |
| 192-223 | C |
| 224-239 | D |
| 240-255 | E |
- Subnets that use mask /24 (255.255.255.0) follow this convention for the four key values within the subnet, with all DDN values in the subnet using the same value for the first three octets:
| Subnet ID | __.__.__.0 |
| First Address | __.__.__.1 |
| Last Address | __.__.__.254 |
| Subnet Broadcast address | __.__.__.255 |
- The subnet ID and subnet broadcast address are reserved and cannot be used as an IP address
Subnet 1 Rulebreakers
Subnet 1 lists the router IP address as 172.16.32.2, with mask /24 assumed. As a result, the subnet uses these values:
| Subnet ID | 172.16.32.0 |
| First Address | 172.16.32.1 |
| Last Address | 172.16.32.254 |
| Subnet Broadcast address | 172.16.32.255 |
The rulebreakers are:
- Address 172.16.31.1 is a valid IP address in a different subnet. It is outside the range of addresses defined by the router’s address/mask combination.
- Address 172.16.32.255 in the correct subnet’s broadcast address, which cannot be assigned to a host to use as its unicast IP address.
Subnet 2 Rulebreakers
Subnet 2 lists the router IP address as 172.16.8.250, with mask /24 assumed. As a result, the subnet uses these values:
| Subnet ID | 172.16.8.0 |
| First Address | 172.16.8.1 |
| Last Address | 172.16.8.254 |
| Subnet Broadcast address | 172.16.8.255 |
The rulebreakers are:
- Address 172.16.8.fe uses hex value “fe” in the last octet. IPv4 DDN values do not support the use of hex values. Instead, each octet must use a decimal value between 0-255, with additional restrictions.
- Address 172.16.8.0 is the subnet ID of the subnet and cannot be used as a host’s unicast IP address.
Subnet 3 Rulebreakers
Subnet 3 lists the router IP address as 172.16.99.199, with mask /24 assumed. As a result, the subnet uses these values:
| Subnet ID | 172.16.99.0 |
| First Address | 172.16.99.1 |
| Last Address | 172.16.99.254 |
| Subnet Broadcast address | 172.16.99.255 |
The rulebreakers are:
- Address 172.16.16.99 is not in the range of addresses in the subnet as defined by the router’s IP address/mask combination.
- Address 172.99.99.99 is not in the range of addresses in the subnet as defined by the router’s IP address/mask combination.