The OSI Model#

The OSI model is one of the fundamental understandings of how networks work. It is a model of 7 layers (or 5 if you’re lucky), with the top layer being the most human-readable, and the bottom layer being purely electronics. When data is transferred across a network, it is all done at the bottom level, but having multiple layers allow us to encapsulate (down) or decapsulate (up) data whereby we don’t need to understand how every layer works.

The Layers#

As mentioned above, there are 7 layers in the OSI model. From top - down, these are:

7 Application Layer
6 Presentation Layer
5 Session Layer
4 Transport Layer
3 Network Layer
2 Data Link Layer
1 Physical layer

Application Layer#

HTTP, FTP, SMTP, POP3, IMAP

Presentation Layer#

SSL, TLS

HTTP 80 HTTPS 443
FTP 21 FTPS 990
SMTP 25 SMTPS 465
POP3 110 POP3S 995
IMAP 143 IMAPS 993

Session Layer#

Transport Layer#

TCP, UDP

Network Layer#

IPv4, IPv6, , ARP, ICMP, routing