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Thread: Different Subnets - Can they talk?

  1. #1
    Jim G
    Guest

    Different Subnets - Can they talk?

    My network is pretty basic.

    Cable Modem to Linksys Wireless Router (handing out dhcp, 4 port)
    (192.168.0.1)
    From wireless router to 8 port netgear firewall (firewall has dhcp turned
    off, firewall off. Acting as a hub right now. Has ip of 192.168.0.17).
    I have a slingbox running on my network and configured as 192.168.0.88. I
    have an issue with my slingbox constantly taking over my router (I have upnp
    disabled as well).
    I want to setup another router with a different subnet, like 192.168.10.x. I
    want to hookup the slingbox to this network to see if this will resolve it
    taking over my 192.168.1.1 router.

    My question is this.

    How would I hookup this new router. Do I go from a lan port on my netgear
    (192.168.0.17) to the internet port on the new router (192.168.1.x), or do I
    go to just a lan port?
    Do I need a cross over cable?
    What about firewall rules to direct traffic to the new router? I have a
    couple servers that I would like to move here too and would need several
    rules in order for them to run (smtp, pop, http, imap etc).
    Would I need some static routes to direct traffic to this new router?

    Please let me know your thoughts.

    Here is my post at the slingmedia site about this issue.
    http://www.slingcommunity.com/forum/...control+router

    Thanks
    Jim


  2. #2
    News Reader
    Guest

    Re: Different Subnets - Can they talk?

    You have not indicated that you are looking to restore the full
    functionality of the firewall, and that it is currently being used as a
    hub. My comments are based on that premise.

    Given your current topology, you would connect the WAN port of the new
    router (the downstream router) to one of the firewalls eight ports. You
    would configure the downstream router's WAN port with an IP address on
    network 192.168.0.0 /24, lets say 192.168.0.2.

    The LAN interface on the downstream router would be configured with an
    address (e.g.: 192.168.1.1) on a different network (e.g.: 192.168.1.0 /24).

    Hosts need to be configured with addresses 192.168.0.x, or 192.168.1.x
    depending on which side of the downstream router they will reside.

    Keep in mind that all IP addresses must be unique. Imagine if you
    mistakenly connected the LAN interfaces of two routers using the same
    factory default IP address, and you tried to connect to the web
    interface of either. You'd be in trouble.

    The new (downstream) router should not need a crossover cable to connect
    to what I presume is an integrated 8 port switch in the firewall.

    You will want to implement static routes (probably best for your
    situation), or dynamic routing (e.g.: RIP) if you prefer, and if the
    routers support it. You do NOT want to be sending dynamic routing
    updates out the WAN interface of the upstream router though!

    The upstream router needs to have a route to 192.168.1.0. (this is
    critical).
    The downstream router already knows about each of your networks because
    it has an interface on each.

    Hosts on 192.168.0.0 will use default gateway 192.168.0.1, per my example.
    Hosts on 182.168.1.0 will use default gateway 192.168.1.1, per my example.

    If you get the routing right, any LAN host can find any other LAN
    host/server regardless of which network they reside on.

    Here is a scenario you should understand:

    Upstream host 192.168.0.10 wants to communicate with downstream server
    192.168.1.20. The host sends its first packet to its default gateway
    (upstream router), which isn't really desirable. The upstream router
    checks its routing table and determines that the next hop router should
    be the downstream router. It will send an ICMP Redirect message to the
    host saying "use the gateway at 192.168.0.2" (i.e.: the downstream
    router). Typically, the upstream router would then forward the packet to
    the downstream router for forwarding to the server.

    The host should add a route to its table, and comply with the advice
    communicated by the upstream router by sending subsequent packets
    directly to the downstream router (i.e.: in the desired direction). I
    have seen hosts that did not comply. They continued to send to the
    upstream router, and then the upstream router forwarded them downstream.
    The result being twice the traffic (undesirable).

    As far as your reference to firewall rules, it is not clear whether you
    are talking about port forwarding on the upstream router, or
    re-establishing full firewall functionality of your Net Gear device
    which would require reconfiguring your topology. Either way, enough for now.

    Best Regards,
    News Reader

    Jim G wrote:
    > My network is pretty basic.
    >
    > Cable Modem to Linksys Wireless Router (handing out dhcp, 4 port)
    > (192.168.0.1)
    > From wireless router to 8 port netgear firewall (firewall has dhcp turned
    > off, firewall off. Acting as a hub right now. Has ip of 192.168.0.17).
    > I have a slingbox running on my network and configured as 192.168.0.88. I
    > have an issue with my slingbox constantly taking over my router (I have upnp
    > disabled as well).
    > I want to setup another router with a different subnet, like 192.168.10.x. I
    > want to hookup the slingbox to this network to see if this will resolve it
    > taking over my 192.168.1.1 router.
    >
    > My question is this.
    >
    > How would I hookup this new router. Do I go from a lan port on my netgear
    > (192.168.0.17) to the internet port on the new router (192.168.1.x), or do I
    > go to just a lan port?
    > Do I need a cross over cable?
    > What about firewall rules to direct traffic to the new router? I have a
    > couple servers that I would like to move here too and would need several
    > rules in order for them to run (smtp, pop, http, imap etc).
    > Would I need some static routes to direct traffic to this new router?
    >
    > Please let me know your thoughts.
    >
    > Here is my post at the slingmedia site about this issue.
    > http://www.slingcommunity.com/forum/...control+router
    >
    > Thanks
    > Jim
    >


  3. #3
    Junior Member gewone's Avatar
    Join Date
    Mar 2008
    Location
    Kingston, Jamaica.
    Posts
    3
    I had just gotten this feeling that I've got the hang of the basics on subnetting, after reading tons of Wikipedia articles, and suchlike. Then this stroke me, how the h-ck could computers from different subnets talk? ...and all of a sudden it felt like I was back on square one.

    However, I did some testing. I have a simple home router. I deactivated its DHCP server, and entered 255.255.255.248 as subnet mask inside the web interface. Rebooted. Then I manually configured IP 192.168.1.6 on one machine, and 192.168.1.8 on the other.

    Funny thing is, then I could ping the two machines from each direction, but I couldn't ping the router itself (192.168.1.1) from the 192.168.1.8 machine. I was dazed and confuzed again, couldn't figure this one out.

    A couple of minutes ago, I figured, after all, I entered 255.255.255.0 as subnet mask in Windoze TCP/IP setting for each of the two computers, perhaps this is the explanation why it worked as in a normal /24 network with the two computers, whereas the router "lived in a little bubble of its own", sort of.

    I when come thinking about it, my router has this cool little PING utility inside its web interface. I should have tried pinging from inside the router aswell, but I guess I would have gotten the same results from that direction (ie. response from .6 but not from .8).

    I'm trying to see the logic now. When the interface (in this case the router) is set to act in eg. an "255.255.255.248 environment", like in this case, it will simply encapsulate its "world as it sees it" into "blocks" of -- in this case -- 8 hosts each.

    Whereas, the two computers (attached to the router) that were instructed to use 255.255.255.0 for subnet mask, hence made blocks of 256 hosts (254 usable) as "the world as they saw it", thus recognized each other even though they were separated in terms of the /29 mask that the router was instructed.

    So basically. I guess the answer is no.
    Computers from different subnets cannot communicate.
    I guess this is how it works, basically..

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