Which layer in the OSI model does a network scan work on?

When doing a network scan using for example NMAP with its "-A" option, what layer of the OSI model does it work on?

For reference, this is the description of the "-A" option: -A : "Enable OS detection, version detection, script scanning, and traceroute"

The OSI model is a theoretical model with 7 layers; there are lots of resources out there describing which layers map to actual protocol layers in various network stacks, so I won't get into that. Instead, I'll give you the breakdown of what happens at each layer of the TCP/IP stack, which has 5 layers.

1. Physical layer. Nmap unavoidably uses this layer, though it is not usually concerned with it. It doesn't matter if you are using Cat 5 cable, 2.4 GHz radio, or coaxial cable—you can't use a network without having a physical layer. Nmap has no idea what it is, either; the firmware in your network card handles that.
2. Data link layer. Here again, Nmap has to use this layer or nothing gets sent to the destination. But there are some cases where Nmap is aware of what layer-2 protocols are in use. These all require root privileges to work:
   • On Windows, Nmap can't send raw IP packets (more on this in the next layer), so it falls back to sending raw Ethernet (layer 2) frames instead. This means that it can only work on Ethernet-like data links—WiFi is fine, but PPTP doesn't work.
   • There are some NSE scripts that probe layer-2 protocols: lltd-discovery, broadcast-ospf2-discovery, sniffer-detect, etc.
   • If the target is on the same data link, Nmap will use ARP to determine if the IP address is responsive. It will then report the MAC address of the target. For IPv6 targets, Neighbor Discovery packets are used instead.
3. Network layer. Nmap supports both IPv4 and IPv6 network layer protocols. For port scans (except -sT TCP Connect scan), Nmap builds the network packet itself and sends it out directly, bypassing the OS's network stack. This is also where --traceroute happens, by sending packets with varying small Time To Live (TTL) values to determine the address where each one expires. Finally, part of the input into OS detection comes from the network layer: initial TTL values, IP ID analysis, ICMP handling, etc.
4. Transport layer. This is where the "port scanner" core of Nmap works. A port is a transport layer address; some of them may be used by services on the target ("open" ports), and others may be unused ("closed" ports). Nmap can scan 3 different transport layers protocols: TCP, UDP, and SCTP. The majority of inputs to OS detection come from here: TCP options, sequence number analysis, window size, etc.
5. Application layer. This is where version detection (-sV) takes over, sending various strings of data (probes) to open services to get them to respond in unique ways. SSL/TLS is handled specially, since other services may be layered over it (in which case it provides something like an OSI Session Layer). This is also where the vast majority of NSE scripts do their work, probing services like HTTP, FTP, SSH, RDP, and SMB.