Older IOS images on Cisco devices used telnet as default login method for the vty lines. Nowadays using telnet is not safe because all traffic including usernames and passwords is transmitted in clear text. This way anybody using a protocol analyzer can gain access to the data. Lately telnet access method has been replaced by SSH which offers increased security by encrypting all traffic between source and destination. SSH protocol comes in two versions SSH1 and SSH2. Communication between the client and server is encrypted in both versions. You should implement SSH version 2 when possible because it uses a more enhanced security encryption algorithm. SSH1 became available in Cisco’s IOS, starting with release 12.1(1)T. In order to use SSH2 you need a IOS version 12.3(4)T or newer.