Re: Query regarding subnetting

From: David Efflandt (efflandt_at_xnet.com)
Date: 10/13/04 

Date: Wed, 13 Oct 2004 00:04:33 +0000 (UTC)

On 12 Oct 2004 03:02:15 -0700, Vardhman <vardhman@gmail.com> wrote:
> This may turn out to be a very naive question, but I am really
> clueless.
>
> Well the situation is like this considers a class C network
> 192.168.39.0 now within this class lets consider a host 192.168.39.60,
> let the subnet of the host be as follows 255.255.255.192 and gateway
> be 192.168.36.1 Now everything is working fine but what will happen if
> I simply change the subnet to 255.255.255.0 only on the host ? Will
> this matter anyway to how the host communicates to the local machines
> or to the gateway ??

Unless you made a typo, that 192.168.36.1 gateway is not in either
subnet. In order to reach it you would need a host or net route for it
before using it as a default gw (unless it is remote ppp IP using
defaultroute option, in which case it is automatic).

> I don't believe a host netmask is known to anyother host by any
> chance, I mean no one can say whether I am in /24 or /28 subnet
> right?? (is it right to call it /24 or /28 domain here ? )

The netmask is used in routing to tell whether a particular route matches
an IP. Then whether that route has a gateway, or not, will determine
whether it uses a gateway to reach that IP or local arp broadcasting.

> How does Router use this netmask ? Do we have to configure this value
> in router at all ?

The router needs to know what IPs to accept from its LAN, and how to reach
IPs on its LAN (or private WAN). If you have an IP on your LAN (or
private WAN) that is not covered by any routes on the router, it would be
unable to reach that IP.

> A subnet AFAIK should matter only in decision whether the packet is to
> be sent to router or directly sent to the host. Am I wrong in my
> thinking this.

Whether an IP is sent to a router or directly (arp) depends upon whether
the first route that matches the IP or IP/netmask has a gateway (or
0.0.0.0 to indicate local).

> Also is the broadcast address shown by ifconfig command used other
> than sending broadcast packets like arp ??

Windows peer2peer file/printer sharing (samba) and Netware use the
broadcast address to find other machines and negotiate master browser.
I don't even know if arp broadcasting uses the broadcast address, it just
does an arp request to find the MAC of a local IP.

Relevant Pages

  • Re: IpTraffic encloded in EthBroadcasts should be routed
    ... The Sender doenst know the gateway IP and also not the ... :Gateway mac. ... Thats the reason why Its an broadcast. ... You mean something like ICMP Router Discovery, ...
    (comp.dcom.lans.ethernet)
  • Re: Arp Broadcast
    ... If I have a X.0.0.0/8 network mask and a station send a packet to the ... How can I evaluate the broadcast load over a subnet like the ARP Broadcast, ... arp would have to match the same network address on the router itself, ...
    (comp.dcom.sys.cisco)
  • Re: How can you get an incomplete ARP?
    ... ARP request and when/if a reply is received, ... "I can ping the broadcast address, ... In order to reply the "target" host must ... The router receives the ARP request from 209:- ...
    (comp.dcom.sys.cisco)
  • Re: How can you get an incomplete ARP?
    ... ARP request and when/if a reply is received, ... "I can ping the broadcast address, ... In order to reply the "target" host must ... The router receives the ARP request from 209:- ...
    (comp.dcom.sys.cisco)
  • Re: Set Up a server using Public IP Addresses on debian
    ... netmask 255.255.255.248 ... broadcast 41.134.19.95 ... Change 'gateway' to be your router IP. ...
    (Debian-User)