Years ago I was asked by a hiring manager in an interview, “Can you explain the OSI 7 layer model”. I knew what OSI was back then, but no I could not.
For the sake of the story, it’s this:
- Layer 7: The application layer.
- Layer 6: The presentation layer.
- Layer 5: The session layer.
- Layer 4: The transport layer.
- Layer 3: The network layer.
- Layer 2: The data-link layer.
- Layer 1: The physical layer.
If you ever come face to face with *that* manager, it’s easier to just remember this as a mnemonic: All People Seem To Need Data Processing
- Application : All
- Presentation : People
- Session : Seem
- Transport: To
- Network: Need
- Data-link: Data
- Physical: Processing
There’s another model known as DARPA named after the research agency that created internet methodologies. Comparatively they look like this with regard to their functional space:
So, protocols are easily organized when you follow the DARPA model, and makes a whole lot more sense if you’re hands on with them because SMTP, SNMP, FTP, DNS, RIP, and Telnet are application driven, therefore application layer. Transport layer being TCP and UDP, no problems there recognizing the conceptual relationship. It’s the Network / Internet layers that have really tested my understanding of the mechanics in networking, IP addressing is fairly straight-forward once you understand subnetting, and functionary tools such as ICMP, IGMP, and ARP. However, the complexity of IP address space is not unlike gardening and irrigation and can to grow to be unmanageable if you’re not engaging in consistent practices.
I came to understand not long after that the interviewer was aiming to tank our discussion with that particular question on OSI, because it had little to do with the actual work being done in this role. He was also married to someone who had some pretty strong emotional attachments to work I had done at a previous job, where I caught her accessing systems via passwords she memorized form other users. That’s a whole other story in and of itself. Still, I wanted to know how to answer that question should it ever come up again. Though, knowing what I know now I might question the motivations of anyone asking me about that in an interview.