IPv4 Address Validation Explained

Understanding the structure of an IPv4 address is key to identifying valid and invalid ones. An IPv4 address is used to uniquely identify a device on a network.

Understanding IPv4 Address Format

An IPv4 address consists of four sets of numbers, called octets, separated by periods (dots). For example, a typical IPv4 address looks like 192.168.1.1.

Each octet must adhere to a specific rule:

• It must be an integer.
• It must be in the range from 0 to 255, inclusive. This can be represented mathematically as $0 \leq \text{octet} \leq 255$.

Analyzing the Options

Let's examine each provided option against these rules:

• Option 1: 202.102.102.11
  • Octets: 202, 102, 102, 11
  • All octets are integers between 0 and 255.
  • Result: Valid IPv4 Address
• Option 2: 121.121.191.191
  • Octets: 121, 121, 191, 191
  • All octets are integers between 0 and 255.
  • Result: Valid IPv4 Address
• Option 3: 21.101.1.1010
  • Octets: 21, 101, 1, 1010
  • The last octet, 1010, is greater than 255.
  • Result: Invalid IPv4 Address
• Option 4: 198.1.1.91
  • Octets: 198, 1, 1, 91
  • All octets are integers between 0 and 255.
  • Result: Valid IPv4 Address

Conclusion on Invalid IP Address

Based on the analysis, the IP address 21.101.1.1010 is invalid because one of its octets (1010) exceeds the maximum allowed value of 255. The other options represent valid IPv4 addresses.