How many bits long is the 'Source IP Address' field in the IPv4 header?

The 'Source IP Address' field in the IPv4 header is 32 bits long. This is because IPv4 addresses are designed to provide a unique identifier for a device on a network and are expressed in a 32-bit format. When represented in decimal, this 32 bits is commonly shown as four octets, each consisting of 8 bits, resulting in a format such as "192.168.1.1". This allows for a total of 2^32 unique addresses, which is approximately 4.3 billion different IP addresses.

In contrast, the other options do not reflect the standard size of an IPv4 address, as they either under-represent or misrepresent the data associated with IPv4 networking. Understanding the bit length is crucial for network configuration and troubleshooting as it directly relates to how devices communicate within the IPv4 addressing scheme.