I've read what I can about IPv4 addressing. My ISP has given me a static address of 209.xx.xxx.255
My confusion is that I guess this is a C class address, but I thought the XXX.XXX.XXX.255 was a broadcast address. Is this actually CIDR addressing? I haven't found an answer on the web yet; Does anyone know where I might find out why my external IP is what appears to be a broadcast address?
You're simply confused by what actually is a broadcast address. You should also forget about the notions of classful networking, as modern networking is largely classless.
With an IP comes its network mask. The network mask defines how long the network part of an IP is, in bits. The rest is the host part. The broadcast address of a network is when the host part of the address is composed of 1 bits, and the network part is left as-is.
Say your network is 209.0.0.0/23 instead of /24. Your network mask is thus 255.255.254.0. The broadcast address would be 209.0.1.255 (every bit including the 24th is flipped). So 209.0.0.255 is a perfectly valid address to hand out.
In many cases, the network mask will be 255.255.255.0 (or /24), but there's no guarantee that this is the case.
Have a good one,