▁specific ▁announ cement ▁of ▁the ▁prefix ▁will ▁be ▁considered ▁invalid . ▁This ▁is ▁a ▁way ▁to ▁en force ▁aggreg ation ▁and ▁prevent ▁hij ack ing ▁through ▁the ▁announ cement ▁of ▁a ▁more ▁specific ▁prefix . ▁ ▁When ▁present , ▁this ▁spec ifies ▁the ▁length ▁of ▁the ▁most ▁specific ▁IP ▁prefix ▁that ▁the ▁AS ▁is ▁author ised ▁to ▁advert ise . ▁For ▁example , ▁if ▁the ▁IP ▁address ▁prefix ▁is ▁ 1 0 . 0 . 0 . 0 / 1 6 ▁and ▁the ▁maximum ▁length ▁is ▁ 2 2 , ▁the ▁AS ▁is ▁author ised ▁to ▁advert ise ▁any ▁prefix ▁under ▁, ▁as ▁long ▁as ▁it ▁is ▁no ▁more ▁specific ▁than ▁/ 2 2 . ▁So , ▁in ▁this ▁example , ▁the ▁AS ▁would ▁be ▁author ised ▁to ▁advert ise ▁ 1 0 . 0 . 0 . 0 / 1 6 , ▁ 1 0 . 0 . 1 2 8 . 0 / 2 0 ▁or ▁ 1 0 . 0 . 2 5 2 . 0 / 2 2 , ▁but ▁not ▁ 1 0 . 0 . 2 5 5 . 0 / 2 4 . ▁ ▁R PK I ▁route ▁announ cement ▁valid ity ▁ ▁When ▁a ▁RO A ▁is ▁created ▁for ▁a ▁certain ▁combination ▁of ▁origin ▁AS ▁and ▁prefix , ▁this ▁will ▁have ▁an ▁effect ▁on ▁the ▁R PK I ▁valid ity ▁of ▁one ▁or ▁more ▁route ▁announ c ements . ▁They ▁can ▁be : ▁▁ ▁VAL ID ▁ ▁The ▁route ▁announ cement ▁is ▁covered ▁by ▁at