If my network’s configured incorrectly, how are my computers communicating?

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IP address
IP address conflicts
LAN
Local area networks
Network Configuration
Network design
Network hardware
Networking
Router IP address
Routers
Routing and switching
I have a question regarding communication within a local network. Please correct me If I am wrong: If there are two computers in a local network, Computer (A) and Computer(B), and if (A) wants to communicate with (B) or vice versa, then the request first strikes the router's LAN interface, right? My question is that if the IP address in the default gateway of the computer which is actually the IP address of the router's LAN interface, is configured incorrectly, then why are the computers in the local network able to communicate with each other? We know that if the default gateway IP address is incorrectly configured then, the computers in the local network cannot communicate outside the network but can communicate internally.
ASKED: June 2, 2009  1:09 PM
UPDATED: June 8, 2009  2:49 PM

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When we have two hosts that belong to the same Physical and Logical network, they should be able to communicate between each other, assuming there is no Firewall or other security software blocking their communication.

The ‘<b>Default Gateway</b>’ option is only required when one of the two hosts are in a different logical network, therefore all packets are sent to the Gateway device (usually a router), that routes packets between the two hosts, making their communication possible.

There are of course many variations of this scenario that raise a lot of questions, and are very helpful to ensure you clearly understand how IP communications work.

For example, assume one host has a configured IP address of <b>192.168.0.5 </b>with Subnet mask <b>255.255.255.0</b> and the other host is configured with IP address <b>192.168.0.90</b> with Subnet mask <b>255.255.255.128</b>.

Are these two hosts able to communicate without the usage of a default gateway ?

In short, the answer is ‘Yes’. Even though at a first glance we see that the subnet masks are different, the truth is that there is a common LAN IP range (192.168.0.1 – 192.168.0.126) between both hosts.

Such examples are often found in certification exams, carefully selected to ensure the user really understands how IP communications, routing and subnetting really works.

You can find plenty of these scenarios at the following URL:

http://www.firewall.cx/ip-subnetting-routing.php

http://www.firewall.cx/ip_routing.php

Hope that helped clear things :)

Cheers,

Chris Partsenidis.

Previous Answer from another user:

For 2 hosts on the same LAN/VLAN to communicate, they do not even need a default gateway.

The router is only used if the host needs to communicate with a different network.

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  • BlankReg
    Nice answer Chris, a proper explaination. Much better than the previous one :-)
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  • Labnuke99
    The real basic answer is ARP (address resolution protocol). Clients will send out ARP broadcasts to find out a mac address of a device on a local LAN (since broadcasts should not cross a router). Since all stations listen to broadcasts, the destination station in question will respond with its mac address and then communication will be directly established between the source and destination hosts.
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