Re: Freebsd 5.1 <-> Win XP Networking problems

From: epilogue (epilogue_at_allstream.net)
Date: 07/15/04

  • Next message: epilogue: "Re: Freebsd 5.1 <-> Win XP Networking problems" 
    Date: Wed, 14 Jul 2004 23:06:38 -0400
To: Danny MacMillan <flowers@users.sourceforge.net>

    On Wed, 14 Jul 2004 16:40:10 -0600
    Danny MacMillan <flowers@users.sourceforge.net> wrote:

    > On Wed, Jul 14, 2004 at 03:41:04AM -0600, Matthew Seaman wrote:
    > > On Tue, Jul 13, 2004 at 02:32:46PM -0700, Darren Pilgrim wrote:
    > >
    > > > > ifconfig_ed0="inet 192.168.1.0/24 netmask 255.255.0.0"
    > > > > ifconfig_vr0="inet 192.168.0.0/24 netmask 255.255.0.0"
    > > >
    > > > 192.168.1.0/24 and 192.168.0.0/24 are blocks of addresses in CIDR
    > > > notation, not the actual addresses to be fed to ifconfig. You need
    > > > to pick addresses within the netblock to use for myserver and all the
    > > > other machines on your network. Since myserver can reach the
    > > > internet just fine, you should keep the IP address for vr0 the same,
    > > > just lengthen the netmask to allow the use of 192.168.1.0/24 on the
    > > > LAN.
    > >
    > > ifconfig(8) understands CIDR notation just fine, although it's not
    > > usual to configure an interface using the '.0' /network/ address. Look
    > > on it as a third alternate way of specifying the netmask, so that the
    > > following three examples are equivalent:
    > >
    > > ifconfig fxp0 inet 192.168.123.74/29
    > > ifconfig fxp0 inet 192.168.123.74 netmask 0xfffffff8
    > > ifconfig fxp0 inet 192.168.123.74 netmask 255.255.255.248
    > >
    > > Those correspond to the slightly contrived example of the /29 network
    > > starting with network address 192.168.123.72 and running up to the
    > > broadcast address 192.168.123.79
    > >
    > > Note: you can give a broadcast address on the ifconfig command line,
    > > but usually it's not necessary as a standard value will be calculated
    > > from any ip number forming part of that network and from the netmask.
    > > However you can't in general use ip address + broadcast to do the
    > > converse, as there isn't necessarily a unique solution.
    >
    > I apologize for asking this question here, but I've googled and read
    > arp(4) and arp(1) and nothing I can see gives a clear answer (at least
    > clear to me). It is related to this thread.
    >
    > Is it the subnet mask that lets my computer know that for an IP address
    > located external to my network it should send the packet to the router
    > (using the router's MAC address) instead of arp-ing for the MAC address
    > of the target node?

    hello danny,

    i'm only going to speak to the part immediately above...

    kind of yes, but mostly no. the subnet mask simple provides a mathematical
    means to segment a single ip block into smaller separate networks.
    technically, the computer will look at it's local network (defined by the
    block and subnet mask) to determine if the target machine is local. if
    not, your machine knows the target machine must be on another network and
    it forwards the packets to the only other place it can, whatever gateway
    you've defined (ie. your local router), which then forwards it up the
    point-to-point connection to its gateway (your ISP's router), which
    continues to forward it based on IP...

    afaik, MAC addresses have nothing to do with this directly. yes, MAC
    addresses (OSI model - data link - layer 2) are mapped to IP addresses (OSI
    model - network - layer 3) and vice versa. these are kept in a cache in
    order to speed up routing, somewhat like having a DNS cache can avoid much
    of the processing wasted on resolving frequently used addresses. generally
    speaking, this cache is volatile in nature and can be cleared manually or
    by power-cycling a router, to provide two examples.

    In case you're curious, this doc is a good primer on IP Addressing and
    subnetting.

    "Understanding IP Addressing: Everything You Ever Wanted To Know"
    http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf

    for more about the ISO model, see google. sorry i don't have an
    interesting link handy.

    hoping that this answered at least part of your question, and crossing
    my fingers that i didn't muddle up any of these details (it has been a
    while since i've looked at this).

    cheers,
    epi

    > This is the only way I can see that this would
    > make sense, as arp -a doesn't seem to return the MAC addresses of boxes
    > on the other side of my router under any circumstances. I read a
    > document online that suggested that a router would recursively ARP for
    > a non-local MAC address but this seems insane and highly improbable to
    > me. More likely is that my computer, knowing that an IP address is
    > not local by examining the network address, would choose a route from
    > its routing table, arp for that router's MAC address, and send the
    > packet thither. But is that what actually happens?
    >
    > Pointers to documentation explaining this accepted with my thanks.
    >
    > --
    > Danny MacMillan
    >
    > _______________________________________________
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