What is the minimum and maximum decimal value that can be represented by an 8-bit binary octet?

0 (00000000) to 255 (11111111) — which is exactly why each IPv4 octet ranges from 0 to 255.

Eight bits give 2⁸ = 256 distinct patterns, which run from 0 to 255 (256 values counting zero) — so an "all-ones" octet is 255, not 256. This is the reason a value like 256 in an IP address is invalid, and why a /24 subnet mask reads as 255.255.255.0. Knowing the cap lets you sanity-check any octet at a glance.

Minimum value: 0

• Binary: 00000000
• All bits are 0

Maximum value: 255

• Binary: 11111111
• All bits are 1
• Calculation: 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255

This is why each octet in an IPv4 address ranges from 0 to 255.

Go deeper:

• Wikipedia — IPv4 § Addressing — why the 0-255 octet cap shapes valid addresses and masks like 255.255.255.0.