InspIRCd has three types of server. Although the same in design, and speaking the same protocol, their behaviours are different due to the roles they play in maintaining the network. These are:
A hub is categorized as any server which has more than one locally connected server. A hub differs from normal operation in that the bandwidth passing through a hub is usually much greater than that passing through a leaf, and it may expend more computing power to run a hub.
A leaf is categorized as any server which has only one, or no connected servers. A leaf does not have to route as much information as a hub (see above) but usually because of this is configured to hold many more clients as it has the spare computer power to hold the users.
A U-lined server is usually a leaf server, however in InspIRCd, a hub may also be U-lined. U-lined servers have many extra privileges, which are required for example to operate a services server or similar. U-lines must be individually configured on all servers to work, and all servers must be in agreement about the names of the U lined servers. If they are not in agreement, mode changes will be dropped and commands forbidden which should operate normally.