A method of providing packet routing information comprises: encoding routing information from a source node to one or more destination nodes into a compact representation of set membership; and putting the compact representation of sets into a header of a packet that is to be sent from the source node to the destination node(s). The compact representation may be obtained by: generating d representations of a set of identifiers; generating d candidate compact representations of set membership from the d representations of the identifiers; and selecting one of the candidate compact representation of set membership. The selection may be made on the basis of which of the candidate compact representations has the lowest rate of returning false positives.