What is the difference between SQL*Net V1, V2, NET8 and NET9?

 SQL*Net V1SQL*Net V2Net8/ Net9
Default port 1525/tcp1521/tcp1521/tcp
Start command tcpctl startlsnrctl startlsnrctl start
Stop command tcpctl stoplsnrctl stoplsnrctl stop
Connect stringprotocol:host:sid eg. T:SRV1:DB1Specified in TNSNAMES.ORASpecified in TNSNAMES.ORA
Config files /etc/oratabtnsnames.ora, sqlnet.ora & listener.oratnsnames.ora, sqlnet.ora & listener.ora
Env variable LOCAL= TWO_TASK=TWO_TASK=