Transport Layer
How do we identify packets of different applications?
We use a 4 tuple which consists of source IP, source port, destination IP, and destination port
Can we reuse ports?
Case 1: Destination Different
If destination IP or port different, then we can use the same source port to establish connection
Case 2: Source Different
If a server is listen to clients and two clients have different source IP or port, then the server can use the same port to establish connection
Well-Known Port Number
| Service | Port | Service | Port |
|---|---|---|---|
| FTP | 20 (data) / 21 (command) | HTTP | 80/8080 |
| SSH | 22 | HTTPS | 443 |
| Telnet | 23 | POP3 (mail client → server) | 110 |
| SMTP (mail server → server) | 25 | IMAP (mail client → server) | 143 |
| DNS | 53 | NFS | 2049 |
| DHCP | 67 (server) / 68 (client) | PPTP(VPN) | 1723 |