You need to check to see what the mask is actually suppose to be - a /24 seems large as well.. See the other one you have with a /29 that is more realistic for IP space an ISP would give you, etc.
I would check with ISP what the settings should be!!!
What I can tell you for sure though - is not suppose to be a /5 ;) That is a freaking HUGE!!!!!! 134 Million addresses!!!
If I had to make an educated guess if that address is assigned to you I would guess either a /30 or /29 would be the most logical.. Those or maybe a /28 would put the wire at .16 and would make sense for your .17 gateway and .18 address.
With that /5 you wouldn't of been able to access any internet site using any of those 134 some million addresses..