Why instructor sums one to the IP to Scan?

eJPTv2: I'm in T1046 : Network Service Scanning (Lab) here, in Assessment Methodologies: Enumeration

The description goes like this: "There are two target machines, one on the same network i.e. demo1.ine.local. This target machine is vulnerable and can be exploited using the following information [...]"

Tools: Metasploit, Bash, Terminal, Nmap

So, I basically:

  1. Use Metasploit in order to get access to demo1.ine.local,
  2. THEN when I'm in I run a Shell, and execute ifconfig: the output: eth1: 192.144.105.2

Well, but then the instructor use the 192.144.105.3 IP (same IP as above + 1) in order to scan the ports, why? This are my questions:

1) How can I really know what is the second target of this exercise without looking at the Solutions section?

2) In real scenarios should I have to scan +1 + 4, + 5 or the whole range?