Re: Routing Tables Mysteries

Discussion in 'MCSE' started by techonline, Feb 9, 2004.

  1. techonline

    techonline Guest

    Hi Experts,
    I am confused about the concept of routing tables. I am a newbie i
    networking. Knows a bit about IP Addressing schemes and how they work
    I know a default gateway is provided for every hosts TCP/IP properties
    I have seen sometimes people create routing tables . What is the nee
    of it. How they are created ? I have just dumped the output of "rout
    Print" .Please help me to understand this . I have asked the sam
    question in one of other forums also. But i m sorry i m still not ver
    clear about the concept of routing tables and in what case we need t
    modify them.
    > route PRINT

    C:\>route print
    ===========================================================================
    Interface List
    0x1 ........................... MS TCP Loopback interface
    0x1000003 ...00 0d 56 50 bd 98 ...... Broadcom 440x 10/100 Integrate
    Controller

    ===========================================================================
    ===========================================================================
    Active Routes:
    Network Destination Netmask Gateway Interface Metric
    0.0.0.0 0.0.0.0 172.25.4.254 172.25.4.141 1
    127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
    172.25.4.0 255.255.255.0 172.25.4.141 172.25.4.141 1
    172.25.4.141 255.255.255.255 127.0.0.1 127.0.0.1 1
    172.25.255.255 255.255.255.255 172.25.4.141 172.25.4.141 1
    224.0.0.0 224.0.0.0 172.25.4.141 172.25.4.141 1
    255.255.255.255 255.255.255.255 172.25.4.141 172.25.4.141 1
    Default Gateway: 172.25.4.254
    ===========================================================================
    Persistent Routes:
    None

    What is a persistent route??

    I will appreciate every bit of information from you .

    Regards.

    techonlin
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    techonline, Feb 9, 2004
    #1
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  2. techonline

    Herb Martin Guest

    > Hi Experts,
    > I am confused about the concept of routing tables. I am a newbie in
    > networking. Knows a bit about IP Addressing schemes and how they work.


    They are just a tad bit archane but once you know the three or four "idioms"
    or "conventions" they are pretty easy to decode.

    > I know a default gateway is provided for every hosts TCP/IP properties.


    Not every, but most.

    > I have seen sometimes people create routing tables . What is the need
    > of it. How they are created ?


    Usually the act of filling out the dialog properties on the NIC creates the
    (basic) table automatically, including the normal "default gateway" entries.

    One a "normal" station you seldom need to create any manual entries;
    creating manual entries on a "router" is much more common, but even there
    isn't always necessary.

    Principle: Every machine routes for "itself". Routers "forward packets"
    for
    other stations.

    If a router or even a machine is directly connected to only ONE (other)
    router,
    then manual entries aren't needed.

    The default gateway (route) entry causes either a machine or a router to
    send all
    remote traffic to the default gateway so with one (other) gateway/router
    there is no
    need for additional routes.

    When a router (or perhaps even a regular) station is BETWEEN two (or more)
    routers then it needs to have routes added. The default gateway points the
    the
    "larger" net (like the Internet) usually, and the additional route(s) point
    to the net(s)
    behing any additional router(s).

    > I have just dumped the output of "route
    > Print" .Please help me to understand this . I have asked the same
    > question in one of other forums also. But i m sorry i m still not very
    > clear about the concept of routing tables and in what case we need to
    > modify them.


    Seldom on a station; more frequently on a router, but only if the machine in
    question is "between" two or more routers. Count your Internet ISP default
    gateway as "one" so if you have no other routers you won't need to add any.

    Count the ISP as "one" so if you have two or more routers the "intermediate"
    routers will need to "know" about the nets "behind" the other router(s).

    ISP------Router1----net2----Router2----net2

    Net2 machines use Router2 as the default gateway; Router2 is connected to
    both net1 and net2 so it doesn't need (explicit) routes to find these --
    they are
    built-in to the routing table automatically.

    Router2 uses Router1 as "default gateway" so it can reach the Internet since
    Router1 uses the ISP as its default gateway, BUT....

    Router1 cannot see "net2" directly and since its default gateway is to the
    left, the
    ISP, net2 is unreachable without an EXPLICIT route so you must add a route
    on Router1 TO net2 by way of Router2

    Conceptually it looks like this:

    route add -p Net2 MASK 255.what.ever.0 Left-NIC.Route.2

    Substitute the appropriate mask, net address for net2, and IP address for
    Left-NIC.

    > > route PRINT

    > C:\>route print
    >

    ===========================================================================
    > Interface List
    > 0x1 ........................... MS TCP Loopback interface
    > 0x1000003 ...00 0d 56 50 bd 98 ...... Broadcom 440x 10/100 Integrated
    > Controller
    >
    >

    ===========================================================================
    >

    ===========================================================================
    > Active Routes:
    > Network Destination Netmask Gateway Interface Metric
    > 0.0.0.0 0.0.0.0 172.25.4.254 172.25.4.141 1


    Send out interface 172.25.4.141 (the machine's address) to the default
    gateway
    at address 172.25.4.254

    The one above is a Default route. The "idiom" is Destination=All Zeros, and
    Mask
    all Zeros means: default gateway.

    (Mask all zeros means "check no bits" and any more specific route will be
    preferred,
    since the rules is LONGEST match is preferred.)

    > 127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1


    This is the route (automatic) that makes "ping 127.0.0.1" always test the
    local machine;
    along with the one 2-down.

    > 172.25.4.0 255.255.255.0 172.25.4.141 172.25.4.141 1


    This route (automatic) is the one for the local net. Out the machine's own
    interface to
    its own "address" is another idiom for "drop the packets" on the local
    wire -- or rather,
    the local subnet can be reached directly with no (true) gateway/router
    required to help.

    > 172.25.4.141 255.255.255.255 127.0.0.1 127.0.0.1 1


    This routes the local machine address back to the same machine on 127 so
    with the
    other it means this address is LOCAL to THIS MACHINE.

    The convention is that ALL 255's means EXACTLY one machine; it's the
    opposite
    of the "all 0's" mask.

    > 172.25.255.255 255.255.255.255 172.25.4.141 172.25.4.141 1


    Broadcast address.

    > 224.0.0.0 224.0.0.0 172.25.4.141 172.25.4.141 1


    Multicast (most people don't use it.)

    > 255.255.255.255 255.255.255.255 172.25.4.141 172.25.4.141 1


    Another (mostly irrelevant) broadcast entry.

    > Default Gateway: 172.25.4.254
    >

    ===========================================================================
    > Persistent Routes:
    > None
    >
    > What is a persistent route??


    One that survives a reboot -- it's "persistent" across boots.

    > I will appreciate every bit of information from you .


    Try that and ask any questions you have.


    --
    Herb Martin
     
    Herb Martin, Feb 9, 2004
    #2
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  3. techonline

    techonline Guest

    Thanks Herb Martin
    Hats off your efforts.
    I sincerely appreciate your efforts to provide me the simplife
    explanation of this complex question.

    Let me rephrase what i have learned.

    We have to modify routing table entries when there are more than on
    router connected to a network (single subnet). If we want to send dat
    to a network which is few networks deep from the current network the
    have to define a way to get to the given host in a network usin
    routing table.

    I have read in a few posts that routing table provides redundant path
    between networks if primary links of getting to the network is down.

    Are routing tables used in VPNs if yes then how . Please cite a
    example .

    Hats off to you efforts and knowledge domain.

    Thanks

    techonlin
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    techonline, Feb 9, 2004
    #3
  4. techonline

    Herb Martin Guest

    "techonline" <> wrote in message
    news:...
    >
    > Thanks Herb Martin
    > Hats off your efforts.


    Thanks. My specialty is simple answers, or at a minimum understandable
    answers, to
    the really complicated questions.

    > I sincerely appreciate your efforts to provide me the simplifed
    > explanation of this complex question.
    >
    > Let me rephrase what i have learned.
    >
    > We have to modify routing table entries when there are more than one
    > router connected to a network (single subnet).


    Frequently, not always. Sometimes just to improve efficiency and not as
    a requirement. It does work the other way around: With ONE router
    exiting the current network you never need more than the default gateway
    entry.

    > If we want to send data
    > to a network which is few networks deep from the current network then
    > have to define a way to get to the given host in a network using
    > routing table.


    Yes, and we can only have ONE ACTIVE default gateway. (We can "list"
    multiple default gateways but that is only for fault tolerance if one goes
    down
    and the alternates will never be use if the first one is working.)


    > I have read in a few posts that routing table provides redundant paths
    > between networks if primary links of getting to the network is down.


    That is the "alternates" I mentioned in the previous paragraph.

    > Are routing tables used in VPNs if yes then how . Please cite an
    > example .


    The EXACT same way in most cases as they are used anywhere else.

    A VPN "interface" once created is treated JUST LIKE a NIC by the OS;
    it accepts routes, filters, etc.

    For instance, I have a Router, with an internal interface (NIC), an external
    interface (NIC) to the ISP's router, and a VPN to a branch office. Let's
    assume that I also have one "Interior router" to my wireless network that is
    withing my larger internal network area.

    One the main router, I will need the following:

    Default gateway setting to the ISP router
    Explicit route to the wireless that points to the interior router
    and that other internal subnet
    Explicit route to the "Branch Office" that indicates the VPN should
    be used.

    One minor difference between a VPN/Dial route and a "normal NIC" is that
    with the normal
    NIC we actually specify the "partner router by IP address" while with the
    VPN/dial we may
    not "know" the partner router (answering router) IP address until we connect
    so the software
    let's us specify the "router" by merely indicating the INTERFACE (and the
    address will be
    calculated automatically on connect.)


    > Hats off to you efforts and knowledge domain.


    No problem.

    --
    Herb Martin
    >
    > Thanks.
    >
    >
    > techonline
    > ------------------------------------------------------------------------
    > Posted via http://www.mcse.ms
    > ------------------------------------------------------------------------
    > View this thread: http://www.mcse.ms/message374188.html
    >
     
    Herb Martin, Feb 10, 2004
    #4
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