IS-IS is not inherently an IP Routing protocol. Its control packets do not use IP. RFC 1195 specifies the use of IS-IS for IP Routing.
The ExtremeXOS software implementation of IS-IS requires that at least one IPv4 or IPv6 address be assigned to an interface.
IP addresses and subnets can be assigned independent of area structure provided that neighboring interfaces are on the same subnet. L1 routers exchange directly reachable IP address/mask/metric tuples. When routing in an L1 area, packets are routed via L1 if the destination address is reachable within the area. Otherwise packets are forwarded to the nearest L2 router. L2 routers advertise all IP addresses reachable in their L1 area (if they are a member of one), as well as directly reachable addresses.