Clustering with a distance matrix

Question

I have a (symmetric) matrix M that represents the distance between each pair of nodes. For example,

    A   B   C   D   E   F   G   H   I   J   K   L         


        

Answer 1

One more possible way is using Partitioning Around Medoids which often called K-Medoids. If you look at R-clustering package you will see pam function which recieves distance matrix as input data.