It's actually a CIDR (Classless Inter Domain Routing) mask. Way back when, classful networks were doled out on the byte boundary, e.g. 255.0.0.0 (Class A), 255.255.0.0 (Class B), 255.255.255.0 (Class C) but as the number of autonomous networks grew on the internet, there was a need to segment those networks further and that's where this comes in.
255.255.248.0 is also known as a /21 (meaning there's 21 bits in the network portion of the address). It contains 8 "Class C" aka /24 subnets.
All this is to say, it's just a larger subnet that Cox has segmented for your particular area or concentrator. It has no bearing on performance aside from being in a larger broadcast domain (but broadcasts are probably restricted in someway on the network between devices anyway).