problem with cli example in thred sdk... i cannot ping the same ip addresse continously it keeps on chaging for device which is in the router role

[Liverpool007]

I have programmed two boards(pca10056) with the cli example in thread sdk.I have set the first one as the leader and the second as the child but after pinging the kit the ip addresse of the child changes along with the state (the state changes to router form child) and i cannot ping on the same ip i need to select another ip addresse, why does this happen?does the board change its ip address after few seconds

[Liverpool007]

i have build the program using segger and programmed the board using nrfgo studio