Little confusion about Routing

[maglouais]

Hello, i'm new'ich to networking, can somebody please tell what is it meant by saying that à router separates LANs(1) when, by default, when i set up two LANs physically connected to the same Router the machines respond to other LAN's machines's pings, is it because the router automatically knows where to route when the networks are physically connected to it(2) ?? do we only have to set up two routers for this topology to "separate" the two lans or is there a way to disable routing between the router's physical interfaces (3)?? I just want to understand this without the use of VLANs

thank's in advance.

[maglouais]

Ps : I just want to understand this without the use of VLANs 

[MHM Cisco World]

If there is no router and one client in small network do broadcast, the broadcast will flood to all network in World.

The device that stop this broadcast is router.

That how router separate vlan/lan.

[maglouais]

Thank you very much for the replay, i am aware of the importance of containing the LAN's broadcast, so "separating" LANs means "only" separating their broadcast traffic ? 

[MHM Cisco World · Accepted solution]

Yes, that right.

Router can pass unicast 

But 

Broadcast no.

[maglouais]

Thank you so much, your answers are of great help. 

Thanks a lot.

[MHM Cisco World]

You are so welcome 

[Joseph W. Doherty]

BTW, understand some of @MHM Cisco World's information is "simplified", 100% accurate information is much more involved, and often makes the "simplified" answer incorrect.  (IMO, I often find it difficult to decide how "technical" to get, when answering a question.  [As a rule of thumb - when dealing with complex information, always be wary of "always" or "never", etc.  Real world - "it depends" often arises.])

I only mention this, because often as you learn more about networking, earlier provided information you later discover is "wrong", but again, it's not so much "wrong" as it's often was just "simplified".  (Or perhaps "right" 99% of the time.)

As a sampling, when discussing "broadcast", we often mean we're discussing (logically) a L2 "global" broadcast, or a L3 "global" broadcast, but there's also, in IP, a network broadcast.  Physically, though, we might also discuss broadcast like behavior, e.g. all frames on a hub (or wireless AP), multicast on a L2 switch (w/o IGMP snooping), unicast flooding, and, of course, logical broadcasts being physically broadcast.

So, because of such, saying a router always blocks broadcast, might not always be 100% accurate.  I recall routers can (and might by default) forward network broadcasts.  They might also (I recall) be configured to even forward global broadcast (generally a very, very bad idea, for the reason that @MHM Cisco World noted).

[Joseph W. Doherty · Accepted solution]

". . . what is it meant by saying that à router separates LANs(1) . . ."

Firstly, a router doesn't have to separate LANs, in fact, routers can be part of a (single) LAN (even with multiple connections).

Originally, a LAN, i.e. Local Area Network, denoted a network with all its components "close" together (vs. a WAN, Wide Area Network, or MAN, Metro Area Network).

VLANs (Virtual Local Area Network) on the other hand, do NOT (to my knowledge) have routers within them.  Also, BTW, a LAN might be comprised of multiple VLANs too.

Unfortunately, probably, you might now be further confused.  (If you have specific questions, you might post such, but also suggest you search the Internet for primers on LANs, VLANs, etc.)

What's really important to understand, is networks have L2 segments (logically, all devices share the same "wire") and L3 segments (logically each segment is a different L2 segment, with a L3 "network address").

Devices within the same L2 segment or L2 domain have L2 addresses (e.g. Ethernet MAC).  Devices with a L3 segment have an address comprised of two (or more - but don't want to further confuse) parts (e.g. IP).  One part of which is the "network ID" and the other part which is "host ID" (and even with a L3 address, generally devices still maintain, and use, a L2 address too, but pure L2 networks actually don't need L3 addresses).

A router is a L3 forwarding device.  It's interfaces are assigned L3 addresses.  Anytime (by default) a router receives a (L3) packet, it checks if that packet is directed to its L3 (interface) address, and if not, also checks if the packet is addressed to a different "network ID" than the one used by the receiving interface.  If the latter is true, it will try to "route" (forward) the packet toward the "network ID" the packet has for the packet's destination address.  If the router doesn't know where to forward such a packet, it drops it.

Also (by default) if the router has multiple interfaces, each with their own L3 address (and each using a different "network ID") it "knows" where to forward some packets, i.e. from receiving interface to another of its interfaces.  This is how two L2 networks connected to the same router can ping each other.

"do we only have to set up two routers for this topology to "separate" the two" L2 segments

No.

"is there a way to disable routing between the router's physical interfaces (3)"

Sometimes, yes (e.g. IRB, VRF, ACL, PBR, null routes, etc.)

"I just want to understand this without the use of VLANs"

Notice I haven't used VLANs, in above, but, again, they often correspond to a L2 segment.  However, a L2 segment might not be provided via a VLAN (and for years they were not, i.e. before there were VLANs, there were L2 segments; and still can be).

[maglouais]

Hi @Joseph W. Doherty , this is the kind of answer that i was looking for, thank you for going a bit deeper than @MHM Cisco World, i did actually for a second question if he was speaking about L2 broadcast, he assumed that i'm enough big boy to know that, i was shy to ask him more about as i needed i bit more technical explanation and that cancels my 2 other questions. It is very much more clear for me now especially with you redirecting the focus on L2 and L3 networks/segments/layers/domains, what confused me is the trivial aspect of saying that "routers separate LANs" when its true to say that "routers link LANs" too, thank you very much @Joseph W. Doherty  you're an amazing person, for a while i though you used "segments" to describe PDUs i forgot that L2 PDU is actually a Frame ':) ':) thank's dude you're awesome. Many thaks @suppy 

[MHM Cisco World]

You in interview and someone ask you  how router separate LAN, What will be your answer ?

if I am in this interview I will answer same what I answer you, broadcast. 
and sure you must read and learn from other answer, and sure other give detail more than me, and sure sure other better than me. 
in this field never say I know everything.

[Joseph W. Doherty]

@suppy you're most welcome.

@MHM Cisco World, I presume, like I, might struggle with how detailed to get.  MHM is solid contributor, and if you had asked more questions (no need to be "shy" on forums, asking questions is what they are for), I'm sure he would been able to further clarify.

[suppy]

Hello maglouais,

in addition to all that have been said, each LAN is expected to have a network address. In such a way that every workstation within thesame LAN can reach one another. However, devices in one LAN can only communicate with those on other LANs if a router exists. For example, if all users in an organization were to be able to send emails to one another, then, they must all have thesame network ID. This way, they can all be connected to a switch and everything will work perfectly. But assuming the IT department belongs to a different network segment, the only way it can communicate with devices on other network segments is through a router. The router will router traffic from one network to another. I hope this helps.