Question: Okay, suppose that I have 2 computers: `Computer A` and `Computer B`, connected with this setup:

```
A->switch->router->internet
          /
B->router/

```

and the closest router to `computer B` is configured to block connections from `Computer A`s switch.

If `computer A` is desperately trying to attack `computer B`, sniffing packets, MiTM attacks, etc., how much bigger/smaller attack surface does it have than the usual

```
Computer A ->router->internet
           /
Computer B/

```

Answer: 
It depends on what type of MitM attack you want to do. 

`arp` spoofing or any type of attack which relies upon Layer 2 will not work across routing domains. 

However, you can perform other attacks such as modifying a host file, [DNS spoofing](http://www.windowsecurity.com/articles-tutorials/authentication_and_encryption/Understanding-Man-in-the-Middle-Attacks-ARP-Part2.html), or other attacks at the network level. You could attack the router itself so all traffic is directed to you, etc. You could trick the victim into installing malware which will redirect all requests to your server first.

If your specific host is being blocked at any point, you can use another machine or compromise another machine to try to find a valid path to your victim.
The basic rule is that on the same ethernet network, IP address spoofing is possible, usually by performing ARP spoofing.

In your second scenario, computer B is completely vulnerable to computer A. Well, unless there is protection at a higher layer, e.g. using SSL.

In this scenario, computer B is NOT vulnerable to computer A:

```
Computer A --> Router -\
                        >- Internet
Computer B --> Router -/

```

Of course, this assumes that both the routers are trusted.

In your first scenario, computer B is still vulnerable. Although it is on a separate ethernet network, the problem is that B's router is on the same ethernet network as A. So although A can't directly spoof B, it can spoof B's router, which still allows A to MITM.