What is subnetting within an octet and how does it differ from octet-boundary subnetting?

Subnetting within an octet means the mask boundary falls partway through an octet (e.g. /25–/30), so one octet is split between network and host bits — unlike octet-boundary subnetting, you must work within the magic-number block size.

Subnetting Within an Octet:

When the subnet mask doesn't fall on an octet boundary (not /8, /16, /24).

Common Within-Octet Masks (4th octet):

Example: 10.0.0.0/8 subnetted to /16

• Subnetting in 2nd octet
• 2^8 = 256 subnets (borrowed 8 bits)
• 2^16 - 2 = 65,534 hosts per subnet

Example: 172.16.0.0/16 subnetted to /24

• Subnetting in 3rd octet
• 2^8 = 256 subnets
• 2^8 - 2 = 254 hosts per subnet

Go deeper:

• Subnetting Mastery (PracticalNetworking) — handling masks that fall partway through an octet.
• Classless Inter-Domain Routing (Wikipedia) — variable-length prefixes that break the octet boundary.