IIS (and other web/application servers) can serve several sites at once from the same IP as long as it can distinguish one site from the others in any request that comes through. To achieve this they should have different IPs or the same IP with at least one domain name associated with it.
However, you can have just one site in your server assigned to one IP without any domain name associated to it, and it can be matched too when a request to that IP comes through.
So, the only thing you need to do is going to that site in IIS and in the"Bindings" dialog make sure it has the IP assigned and no domain name:
Before that make sure no other site in IIS has the same Ip assigned without a domain name because in that case IIS has no way to know which site should respond to requests in that IP.