In a packet radio network a packet data address is activated for a terminal for transmitting data packets between the terminal and an external network. Information on the activated packet data address is stored at least in the edge nodes of the network. To prevent spoofing, i.e. misrepresentation of sender data, the method and network node of the invention comprise checking (206) in the node whether the source address of the packet transmitted from the terminal is the same as the packet data address used in the transmission of the packet or does the source address belong to a set of allowed packet data addresses. The packet is transmitted (207) from the node towards the destination address only if the addresses are identical or the source address belongs to the set of allowed packet data addresses.