zookeeper启动后，为何会监听两个端口？

./zkServer.sh start
启动zookeeper后，用ps看进程号是4377
然后

$ netstat -nlp|grep 4377
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
tcp6 0 0 :::2181 :::* LISTEN 4377/java
tcp6 0 0 :::42860 :::* LISTEN 4377/java

发现监听两个端口
怎么回事？