Ip – Connection between 2 end devices with different subnet masks (Packet Tracer)

It's likely that the subnet masks are throwing you off. As long as you keep in mind that the below rules no longer apply, you should be fine.

Ultimately classful addressing came down to the most significant (or "leading") bits in the address. Nothing more, nothing less.

• Class A: Most significant bits starts with 0
• Class B: Most significant bits start with 10
• Class C: Most significant bits start with 110

The "classes" came from the way they split up the address space for use between "host" and "network". Keep in mind that back then (way way back, from the days of ARPANET), subnet masks did not exist, and the network was intended to be inferred from the address itself. So, with the above in mind, this is what they came up with (this is intended to be binary representation - each N or H represents a single bit in the 32-bit address):

• Class A: NNNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH (less networks, more hosts)
• Class B: NNNNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH (more networks, less hosts)
• Class C: NNNNNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH (even more networks, even less hosts)

Here the N is representative of the network portion of the address, and the H is representative of the host portion of the address, or as they called it back in the day, the "rest field."

Combining that with what was said earlier about the most-significant bits, we have the following:

• Class A: 0.0.0.0 - 127.255.255.255
• Class B: 128.0.0.0 - 191.255.255.255
• Class C: 192.0.0.0 - 223.255.255.255

Converting those ranges to binary may make this more clear:

Class A

0.0.0.0
-----------
[0]0000000.00000000.00000000.00000000

127.255.255.255
-----------
[0]1111111.11111111.11111111.11111111
 ^
 most significant bit = 0

Class B

128.0.0.0
-----------
[10]000000.00000000.00000000.00000000

191.255.255.255
-----------
[10]111111.11111111.11111111.11111111
 ^
 most significant bits = 10

Class C

192.0.0.0
-----------
[110]00000.00000000.00000000.00000000

223.255.255.255
-----------
[110]11111.11111111.11111111.11111111
 ^
 most significant bits = 110

Every single address within those ranges will share a common leading bit(s). The moral of the story is, if you can remember what the leading bits are supposed to be (0 for class A, 10 for class B, 110 for class C) it's extremely simple to determine what "class" an address would have otherwise belonged in. Or, if decimal is easier:

• Class A: First octet in address is between 0 and 127, inclusive
• Class B: First octet in address is between 128 and 191, inclusive
• Class C: First octet in address is between 192 and 223, inclusive

The easiest way to mess someone up on "classful addressing" either on a test, or exam, or whatever, is to use misdirection by way of a subnet mask. Again, remember that the subnet mask does not apply for determining the class of an address. This is easy to forget because as others have said, classless addressing and routing have been around for over two decades now, and the subnet mask and CIDR notation have become ubiquitous in the industry.

First of all that You have route from PC0 to PC1 doesn't mean that You have route from PC1 to PC0.

Thats are Zinin’s 3 routing principles:

1. "Every router makes its decision alone, based on the information it has in its own routing table.“
2. "The fact that one router has certain information in its routing table does not mean that other routers have the same information.“
3. "Routing information about a path from one network to another does not provide routing information about the reverse, or return path."

That means all routers must have full routing table to all networks - R1 to networks beyond R3 and R3 to all networks beyond R2.

Besides R3 probably don't have any information about networks at the other site of R2. You can check it showing R3 routing table - show ip route