The present invention relates to a method and an arrangement in communication systems utilizing network mobility (NEMO). In moving networks comprising a plurality of mobile routers, MRs, it needs to be assured that two or more MRs sharing the same prefix, belong to the same moving network. The method according to the invention utilizes a local connectivity test to determine if a MR should be allowed a certain prefix. Unique shared secrets are agreed between the MRs and the home agent, HA. The MRs derive, based on their respective shared secrets, MR specific information, which are distributed among MRs in the moving network. A MR requesting a prefix from the HA, forwards the MR specific information to the HA. By comparing data generated using its stored shared secrets with data from the MR specific information, the HA can determine whether the MR requesting a certain prefix belongs to the same moving network as MRs already using that prefix.