Simple Question

New to the forum* I'm in the beginning phases of studying for the CCENT and have what I assume is a relatively simple question on the topic of router implementation in regards to class C subnetting:

In answering a question posed on page 159 (in reference to figure 3.21, pg 160) of the CCENT Study Guide (Todd Lammle) I was able to understand mask values, block sizes, etc. The question at hand asks why hostA cannot communicate with hostB. After exhausting my theories, which didn't take long, I read on to find the answer that perplexes me still.
::RouterA's serial 0/0 is configured with an IP address in the hostA LAN subnet range::

**While I was able to deduce this from the given info, I was amiss as to why RouterA could not be in the same subnet as hostA.**

Anyone?

Thanks

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Comments

CodeBlox

The Host A is going to likely be on Router A's (fast | gigabit)ethernet interface subnet, not the serial interfaces. That serial interface is typically point to point to another router(Within the scope of CCENT). So they can't be on the same subnet.

EDIT: You could think of it this way, each port on the router should belong do a different subnet (at the ccent level). I say "at the ccent level" because I'm not too sure if that changes. And you aren't going to have a host connected to a routers serial port.

capitanuionut

CodeBlox wrote: »

EDIT: You could think of it this way, each port on the router should belong do a different subnet (at the ccent level). I say "at the ccent level" because I'm not too sure if that changes. And you aren't going to have a host connected to a routers serial port.

It pretty much doesn't change...this is the main principle...what you will see is that a router's interface can belong to multiple subnets...using subinterfaces....but that's not the problem now..

MickQ

The point of a router is to route between different networks (ok, and broadcast isolation). Router A should be on the same network as Host A, but only on its ethernet connections as it uses the serial to route over to the network that Host B is on, right? There wouldn't be much point in having routers between hosts A and B if there's the same network in the middle as on both ends.

I haven't seen the network map for this, but I assume that
Host A is connected to Router A on Router A's E1 port.
Router A is connected to Router B on Router A's S0/0, and Router B's S0/0.
Host B is connected to Router B on Router B's E1 port.

H A <

> (Fa1) ... ... R A ... ... (S0/0) <

> (S0/0) ... ... R B ... ... (Fa1) <

> H B
... ... ... Subnet 1 ... ... ... ... ... ... ... ... ... Subnet 2 ... ... ... ... ... ... ... ... ... Subnet 3

Possible subnets:
Host A and Router A (ethernet): 192.168.1.0 SM: 255.255.255.0 (/24)
Router A and Router B (serial): 192.168.2.0 SM: 255.255.255.0 (/24)
Host B and Router B (ethernet): 192.168.3.0 SM: 255.255.255.0 (/24)

Gives us:
H A <

> (Fa1) ... ... R A ... ... (S0/0) <

> (S0/0) ... ... R B ... ... (Fa1) <

> H B
... ... ... Subnet 1 ... ... ... ... ... ... ... ... ... Subnet 2 ... ... ... ... ... ... ... ... ... Subnet 3
192.168.1.1 ...192.168.1.2 ... ...192.168.2.1 ...192.168.2.2 ... ...192.168.3.1 ...192.168.3.2