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Home > Archive > alt.certification.network-plus > December 2003 > Packet Header Question
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Packet Header Question
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| geomax 2003-11-29, 1:24 pm |
| The packet header contains both source and destination address.
When you send data over the wire to a destination PC, how does your PC know
the destination address? Does your PC maintain an ARP table? Where is it
located?
Can someone describe the process of how the destination address is known?
Thanks!
| |
| Charles M. Kozierok 2003-11-29, 4:24 pm |
| In article <t25yb.12997$UG2.9128@nwrdny03.gnilink.net>,
geomax <gw091@yahoo.com> wrote:
} The packet header contains both source and destination address.
}
} When you send data over the wire to a destination PC, how does your PC know
} the destination address? Does your PC maintain an ARP table? Where is it
} located?
}
} Can someone describe the process of how the destination address is known?
Question is a bit vague. If you mean IP addresses, the PC knows because
you tell it, either directly or by giving it a DNS name that is resolved
to an address.
peace,
-*-
charles
| |
| geomax 2003-11-29, 6:24 pm |
| Lets say there are 3 PC's on the same subnet. Call them A, B, and C.
Computer A wants to sent data to computer C. How does computer A come to
know computer C's address (IP, MAC)? Does it query the subnet for PC's via
ARP? If so, where does the ARP table reside?
Even if you have a router, the router is aware of the nodes attached to it.
But packets coming into the router already have the IP of the destination
computer.
Each PC must be able to determine the addresses of the other PC's. How is
this accomplished?
g
"Charles M. Kozierok" <ixlu@PCGuide.com> wrote in message
news:bqb1sc$g18$1@reader2.panix.com...
> In article <t25yb.12997$UG2.9128@nwrdny03.gnilink.net>,
> geomax <gw091@yahoo.com> wrote:
> } The packet header contains both source and destination address.
> }
> } When you send data over the wire to a destination PC, how does your PC
know
> } the destination address? Does your PC maintain an ARP table? Where is it
> } located?
> }
> } Can someone describe the process of how the destination address is
known?
>
> Question is a bit vague. If you mean IP addresses, the PC knows because
> you tell it, either directly or by giving it a DNS name that is resolved
> to an address.
>
> peace,
>
> -*-
> charles
| |
|
| It is first checking it's Resolver Cache (where also the hosts file is
loaded). If it find no match it is using DNS to resolve the IP address for
the given computer name.
If DNS Name resolution fails and Wins is enabled it will try with that and
start over again with some more options.
Do a search in your help files (for DNS, WINS and Hosts) to get a good
start.
The ARP is only used to keep track of the Physical Address for a specific IP
Address and you can find (and make changes) in the registry with arp.exe
from your command prompt.
AT
"geomax" <gw091@yahoo.com> wrote in message
news:EK9yb.10975$i81.1408@nwrdny02.gnilink.net...
> Lets say there are 3 PC's on the same subnet. Call them A, B, and C.
>
> Computer A wants to sent data to computer C. How does computer A come to
> know computer C's address (IP, MAC)? Does it query the subnet for PC's via
> ARP? If so, where does the ARP table reside?
>
> Even if you have a router, the router is aware of the nodes attached to
it.
> But packets coming into the router already have the IP of the destination
> computer.
>
> Each PC must be able to determine the addresses of the other PC's. How is
> this accomplished?
>
> g
>
> "Charles M. Kozierok" <ixlu@PCGuide.com> wrote in message
> news:bqb1sc$g18$1@reader2.panix.com...
> know
it[color=blue]
> known?
>
>
| |
| geomax 2003-11-29, 8:24 pm |
| What if your network does not use DNS, and WINS is not enabled? For example,
at my company, we use static IP.
"AT" <torgny@@direcway.com> wrote in message
news:0Layb.21911$sb4.16678@newsread2.news.pas.earthlink.net...
> It is first checking it's Resolver Cache (where also the hosts file is
> loaded). If it find no match it is using DNS to resolve the IP address for
> the given computer name.
> If DNS Name resolution fails and Wins is enabled it will try with that and
> start over again with some more options.
> Do a search in your help files (for DNS, WINS and Hosts) to get a good
> start.
> The ARP is only used to keep track of the Physical Address for a specific
IP
> Address and you can find (and make changes) in the registry with arp.exe
> from your command prompt.
>
> AT
>
> "geomax" <gw091@yahoo.com> wrote in message
> news:EK9yb.10975$i81.1408@nwrdny02.gnilink.net...
via[color=blue]
> it.
destination[color=blue]
is[color=blue]
PC[color=blue]
is[color=blue]
> it
because[color=blue]
resolved[color=blue]
>
>
| |
| Charles M. Kozierok 2003-11-29, 9:24 pm |
| In article <EK9yb.10975$i81.1408@nwrdny02.gnilink.net>,
geomax <gw091@yahoo.com> wrote:
} Lets say there are 3 PC's on the same subnet. Call them A, B, and C.
}
} Computer A wants to sent data to computer C. How does computer A come to
} know computer C's address (IP, MAC)? Does it query the subnet for PC's via
} ARP? If so, where does the ARP table reside?
If computer A wants to send date to C, it must have its IP address.
Either the user supplies it, or they supply a domain name that is
resolved to an IP address using DNS or a host table.
The IP address is changed to a MAC address using ARP. Each device
maintains an ARP cache table.
If you could give a more specific example of exactly what transaction you are
trying to figure out...
peace,
-*-
charles
| |
| geomax 2003-11-29, 10:24 pm |
| Take a look at this example:
http://tutorials.beginners.co.uk/im.../454/image1.gif
There are 2 PC's: 192.168.1.1 and 192.168.1.2
IP uses ARP to get the MAC address from 192.168.1.2 - BUT, how did
192.168.1.1 know the address 192.168.1.2 to begin with??
Also, say the first pc has the network ID JSMITH, and the other BJONES.
Without using Netbios or DNS, (IP's are static), how do PC's resolve IP
address to the Network ID?
"Charles M. Kozierok" <ixlu@PCGuide.com> wrote in message
news:bqbhui$kc6$1@reader2.panix.com...
> In article <EK9yb.10975$i81.1408@nwrdny02.gnilink.net>,
> geomax <gw091@yahoo.com> wrote:
> } Lets say there are 3 PC's on the same subnet. Call them A, B, and C.
> }
> } Computer A wants to sent data to computer C. How does computer A come to
> } know computer C's address (IP, MAC)? Does it query the subnet for PC's
via
> } ARP? If so, where does the ARP table reside?
>
> If computer A wants to send date to C, it must have its IP address.
> Either the user supplies it, or they supply a domain name that is
> resolved to an IP address using DNS or a host table.
>
> The IP address is changed to a MAC address using ARP. Each device
> maintains an ARP cache table.
>
>
> If you could give a more specific example of exactly what transaction you
are
> trying to figure out...
>
> peace,
>
> -*-
> charles
| |
| BigDog 2003-11-30, 1:24 am |
| Would not broadcasts be the answer you are looking for, since they are
on the same subnet?
On Sun, 30 Nov 2003 02:51:04 GMT, "geomax" <gw091@yahoo.com> wrote:
>Take a look at this example:
>http://tutorials.beginners.co.uk/im.../454/image1.gif
>
>There are 2 PC's: 192.168.1.1 and 192.168.1.2
>
>IP uses ARP to get the MAC address from 192.168.1.2 - BUT, how did
>192.168.1.1 know the address 192.168.1.2 to begin with??
>
>Also, say the first pc has the network ID JSMITH, and the other BJONES.
>Without using Netbios or DNS, (IP's are static), how do PC's resolve IP
>address to the Network ID?
>
>
>"Charles M. Kozierok" <ixlu@PCGuide.com> wrote in message
>news:bqbhui$kc6$1@reader2.panix.com...
>via
>are
>
| |
| Charles M. Kozierok 2003-11-30, 11:25 am |
| In article <sWcyb.16964$lF6.8399@nwrdny01.gnilink.net>,
geomax <gw091@yahoo.com> wrote:
} Take a look at this example:
} http://tutorials.beginners.co.uk/im.../454/image1.gif
This graphic is illustrating the operation of the Address Resolution
Protocol (ARP). The device on the left broadcasts an ARP Request
containing the IP address it is trying to resolve, and the one on the
right responds with an ARP Reply containing its MAC address.
} There are 2 PC's: 192.168.1.1 and 192.168.1.2
}
} IP uses ARP to get the MAC address from 192.168.1.2 - BUT, how did
} 192.168.1.1 know the address 192.168.1.2 to begin with??
It was either supplied by the user who entered whatever application
command led to this transaction, or it was resolved from a DNS or
NETBIOS name.
} Also, say the first pc has the network ID JSMITH, and the other BJONES.
} Without using Netbios or DNS, (IP's are static), how do PC's resolve IP
} address to the Network ID?
I don't understand what you are asking here. "JSMITH" and "BJONES" are
NETBIOS names. They have to be resolved *to* IP addresses. They are then
resolved to MAC addresses using ARP.
peace,
-*-
charles
| |
| geomax 2003-11-30, 12:24 pm |
| That's it. Thats what I was after. Thanks!
>"JSMITH" and "BJONES" are
> NETBIOS names. They have to be resolved *to* IP addresses. They are then
> resolved to MAC addresses using ARP.
>
>
> peace,
>
> -*-
> charles
| |
|
| I am not sure what you are looking for but, When computer B boots it tells
or try to tell everyone its name and who it is. That's how the routers and
other computers know. Remember, there are routing tables on anything that
routes and it looks at what it is connecting to, if it does not see a match
it sends it to its outbound next hop so it can find it.
If that doesn't help try reading a bit about routing tables at a high level
and that may shed a little more light.
"geomax" <gw091@yahoo.com> wrote in message
news:sWcyb.16964$lF6.8399@nwrdny01.gnilink.net...
> Take a look at this example:
> http://tutorials.beginners.co.uk/im.../454/image1.gif
>
> There are 2 PC's: 192.168.1.1 and 192.168.1.2
>
> IP uses ARP to get the MAC address from 192.168.1.2 - BUT, how did
> 192.168.1.1 know the address 192.168.1.2 to begin with??
>
> Also, say the first pc has the network ID JSMITH, and the other BJONES.
> Without using Netbios or DNS, (IP's are static), how do PC's resolve IP
> address to the Network ID?
>
>
> "Charles M. Kozierok" <ixlu@PCGuide.com> wrote in message
> news:bqbhui$kc6$1@reader2.panix.com...
to[color=blue]
> via
you[color=blue]
> are
>
>
| |
| Barry Watzman 2003-12-07, 1:25 pm |
| If your PC has communicated with the destination before, it may already
have the destination address in a table.
If not, there is a special type of packet that is sent out for the
purpose of getting the destination address before the "actual"
transaction packet is sent out.
The header contains both the source and destination IP address and also
the source and destination MAC address. That's how it gets to the right
PC if the PC is one of multiple PC's behind a router using a private IP
address.
geomax wrote:
> The packet header contains both source and destination address.
>
> When you send data over the wire to a destination PC, how does your PC know
> the destination address? Does your PC maintain an ARP table? Where is it
> located?
>
> Can someone describe the process of how the destination address is known?
>
> Thanks!
>
>
| |
| Barry Watzman 2003-12-07, 1:25 pm |
| I think that you are confusing DNS with DHCP. You MUST have a DNS
server to resolve names (www.microsoft.com) to IP addresses
(207.46.134.221), unless you just enter the numeric IP address itself
every time. However, before actual communication can be established,
you will also need the MAC address of the actual network card in the
destination computer as well, since 207.46.134.221 is likely not the
computer but a router, behind which lie many computers, including the
one that you want to get to. A special packet (ARP - address request
packet) is sent out to get that information before the actual "data"
exchange takes place.
So DNS goes from name to IP address, then an ARP packet is sent out to
the IP address to get the MAC address, then you have everything
required. This information IS cached locally once it's obtained, but
the limits of the local caches are finite and relatively small.
geomax wrote:
> Take a look at this example:
> http://tutorials.beginners.co.uk/im.../454/image1.gif
>
> There are 2 PC's: 192.168.1.1 and 192.168.1.2
>
> IP uses ARP to get the MAC address from 192.168.1.2 - BUT, how did
> 192.168.1.1 know the address 192.168.1.2 to begin with??
>
> Also, say the first pc has the network ID JSMITH, and the other BJONES.
> Without using Netbios or DNS, (IP's are static), how do PC's resolve IP
> address to the Network ID?
>
>
> "Charles M. Kozierok" <ixlu@PCGuide.com> wrote in message
> news:bqbhui$kc6$1@reader2.panix.com...
>
>
> via
>
>
> are
>
>
>
>
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