Overlap check
Check whether two networks share any addresses, and where.
10.0.0.0/16 · 65,536 addresses
10.0.5.0/24 · 256 addresses
10.0.5.0/24 sits entirely inside 10.0.0.0/16. Fine if A is a summary or default route; a problem if both are subnets you assigned.
Shared: 10.0.5.0 – 10.0.5.255 (256 addresses)
On one axis
Both blocks drawn to scale on the same address line
When is overlap a problem?
Two CIDR blocks either do not overlap at all, or one sits completely inside the other. They can never partly overlap, because every block starts on a boundary of its own size.
A problem: two subnets in your design that overlap, like the same 10.1.1.0/24 on two VLANs, or a /23 typed where a /24 was meant. Hosts in the shared part may be reachable from only one side, and routers face ambiguity: they follow the longest match, which may not be the site you expected. Duplicate addresses also break ARP.
Intended: a summary route that contains its more-specific routes, like 10.0.0.0/16 advertised to the core while 10.0.5.0/24 exists inside it. Longest-prefix match sends traffic for 10.0.5.x to the /24 and everything else to the /16. The same goes for a default route 0.0.0.0/0, which overlaps everything on purpose.