How most of the dedicated server hosting providers offer a 100% uptime guarantee on their network.

For providing the 99% uptime, dedicated server providers include a SLA (Service Level Agreement) based on network uptime.
By securing multiple vendors for connectivity and using redundant hardware, providers are able to guarantee higher uptimes; usually between 99-100% uptime if they are a higher quality provider.
One aspect of higher quality providers is they are mostly likely multi-homed across multiple quality uplink providers, which in turn, provides significant redundancy in the event one goes down in addition to potentially improved routes to destinations.