Well, SNMP is pretty great. There's three variants in common use, v1, v2c, and v3. I'm a big fan of v2c, because I usually run SNMP over my trusted LAN, and read only, so there's little or no risks there. I just want all the information! Haha I would consider v3 if I was doing any kind of read/write work with SNMP. To date, I've never had to, so I just don't bother with it. It's a bear to set up compared to v2c.

ARP is on layer 2/3 of both the OSI model and the 5 layer TCP model. The OSI model has never been implemented in a production network, it's just a reference to visualize how things operate. TCP/IP and ipv6 generally stop around the OSI model layer 5. 6/7 is handled by the software, in theory, and layer 8 is where you get the most problems, by far.

ARP is considered to be both layer 2 and layer 3, sometimes noted as layer 2.5, because it's bridging layer 2, which is Ethernet Mac addressing in most networks, and layer 3 which is IP addressing. It almost entirely operates on layer 2 however.

There's a new, revised version of the TCP model that I'm aware of that blurs the line between what is known as layer 1 and 2 in the OSI model, kind of bundling them together. It's weird, but something I've seen around.

The question I never got an answer to was about Ethernet. I have searched the internet high and low and have yet to find a credible reference that indicates what the real answer is. There's a white paper but you have to pay to see it, I'm pretty sure the answer is in there, obfuscated by some fancy math algorithm.... The question is: how much voltage is used for Ethernet baseband signaling when PoE is not used? What constitutes a "high" signal, and what is a "low" signal? A lot of sources seem to point to 5v and 1v, but never have any references to back up the claim. There are other sites that provide different voltages for high and low too. 5/1 is just the most common that I've seen mentioned.