[Novalug] simple route issue - howto

David J jensen.clan@gmail.com
Sat Dec 31 17:06:12 EST 2016


Great and appreciated 101  networking lesson. Thanks for taking the time to
write and share it.

Thanks
David


On Dec 31, 2016 1:51 PM, "Stephen Cicirelli via Novalug" <
novalug@firemountain.net> wrote:

> This is a somewhat long and simplified accounting on some basic
> networking that I hope will help.
>
> The simplistic and unhelpful answer to Walt's question is you need a
> 3rd device (or much more complex and advanced: you need two IP
> addresses on the computer assuming all computers in question are on
> the same switch/wireless SSID).
>
> This should show why the 3rd device is needed
>
> Computer alpha has:
> IP 192.168.1.10
> mask 255.255.255.0 (also called /24 indicating that there are 24 bits
> masking the ip)
> The IP is broken down into network and node, the mask tells you what
> is what.  Since /24 is the easy case (and relevant) we will stick with
> that.
>
> so:
> 192.168.1 is the network
> 10 is the node address
>
> in the real world: 1020 main st., fairfax va
> main st., fairfax va is the network
> 1020 is the node address
>
> When you are on the same subnet as another host (1-254 in this case)
> you don't route anywhere you just talk on the line.  Much like if you
> want to go to the neighbors house at 1022 you just walk next door.  No
> car or on ramps (routes)
> Thus you can :ping 192.168.1.x and provided that x exists and is
> running you will get a response.
>
> The same is true for systems on the other subnet - 192.168.2.20 /24
> computer beta has 192.168.2.20 255.255.255.0
>
> 192.168.2 is the network
> 20 is the node address
>
> in the real world: 2020 elm st. fairfax, va
> elm st., fairfax va is the network
> 2020 is the node address
>
> beta can talk to any computer on 192.168.2.[1-254] without routing
> one could walk to any house on the street and visit 2022 without
> getting into the car.
> Again, ping 192.168.2.y will get a response if the y node is up and running
>
> IF I try: Alpha: ping 192.168.2.20
> I will get either a request timed out or a destination host
> unreachable becuase I don't know how to get there from here.
>
> This is where routing comes in.
>
> routing is how you get from subnet A (192.168.1.0/24) to subnet B
> (192.168.2.0/24)
> (the zero at the end indicates the start of the subnet (which is
> always 0 for a /24) and for now is helpful notation.  This number
> becomes more important with advanced subneting (also refereed to as
> CIDR))
>
>
> for simple subnet scenarios that most people encounter at home
> (192.168.1.0) the nodes do not need a route - they just need a default
> route (given by the default gateway) - because everything is out the
> default gateway.  In our real world analogy this would be the traffic
> light at the end of the street (and of course the only thing here in
> the neighborhood is main st.)  and the light would be the router.
> Typically the router IP address on the subnet (aka default gateway) is
> either the first or the last IP (so 1 or 254 in our case).  (now you
> see why I used 10 and 20)
>
> So now we need a router.  Let's call it delta.  It has two ethernet ports.
> ETH0 - 192.168.1.1
> EHT1 - 192.168.2.1
>
> (as a side note, a modern professional class router can handle
> multiple IP addresses on a single NIC, but your average linksys or
> dlink with the default firmware cannot)
>
> So I need gateways on alpha and beta.
> alpha:
> IP 192.168.1.10
> Mask: 255.255.255.0
> Gateway: 192.168.1.1
>
> beta:
> IP 192.168.2.20
> Mask: 255.255.255.0
> Gateway: 192.168.2.1
>
> So now when I'm on computer alpha and I:
> ping 192.168.2.20
> I will get a response.  computer alpha will see that 192.168.2 is not
> part of its current subnet of 192.168.1 and so the IP stack will send
> the packet to the default gateway (192.168.1.1) who will then route it
> to the 192.168.2 subnet (on eth1) and it will be sent to the
> destination.  The return traffic goes the same way: Beta sees that
> 192.168.1 is not on its network and so sends the packet to 192.168.2.1
> who then handles the routing to send the response.
>
> Thus far we have not needed to add routes.  Indeed, the only one that
> needs a route is the router.
>
> Back to our real world analogy - I live on main st. and I want to
> visit my friend who lives on elm.  I need to get in the car and drive
> to the trafficlight and then the GPS takes me to the traffic light
> that has elm and drops me there.  I then get out of my car and wander
> down the (nice pedestrian friendly) street screaming my fool head off
> looking for my friend.  When I want to go back home, I get back in the
> car and go to the router - er, trafficlight and GPS my way back to
> main st.
>
> So why would one need to use route?  Typically one wouldn't.
> Typically most homes are on a single network, as we said above, and
> the entire universe is out the default gateway.  That's all the system
> needs to know.
>
> Here is where is gets complex without pictures.
>
> Let say we have computer gamma:
> 192.168.3.30/24 GW:192.168.3.1
> so this one is set up just like the others on a per subnet basis.
>
> let us also say we have a new router epsilon
> ETH0 192.168.3.1/24
> ETH1 192.168.1.2/24 <-- note the subnet here is on 192.168.1 and the node
> is 2
>
> so epsilon talks to and routes subnets with alpha and gamma (no beta)
>
> So what happens when alpha does: ping 192.168.30.30
> It fails.  Because alpha sends the packet to router gamma (192.168.1.1
> - the default gateway) and it doesn't go anywhere (and yes, there are
> advanced routing configurations that would fix it)
>
> This is where route would come into play.
> You would put in something like:
> route ADD -net 192.168.3.0 MASK 255.255.255.0 192.168.1.2
> on alpha (note you don't need to do anything for gamma)
> This would tell alpha
> If you have traffic that isn't for 192.168.1.x then send it to the
> default gateway (192.168.1.1) UNLESS it is traffic for subnet
> 192.168.3.w, then send it to 192.168.1.2
>
> I hope this little tutorial on networking helps folks in general.  As
> usual please ask clarifying remarks and I hope everyone has a fabulous
> 2017
> Happy New Year!
>
> -Stephen
>
>
> On Fri, Dec 30, 2016 at 5:45 PM, Walt Smith via Novalug
> <novalug@firemountain.net> wrote:
> >
> >
> > Hi,
> >
> > I have a simple route problem.   I'm probably
> > missing just one thing.  TO approach that,
> > I merely tried a number of combinations that
> > theory doesn't seem to cover when actually
> > using commands.  It using static routing.
> >
> > ( note most web blogs etc discuss routing between
> > eth0 and eth1, which this is not )
> >
> > The idea is I CAN assign 192.168.1.1 to the
> > eth0 interface, and route with netmask 255.255.255.0
> > and access any devices  on several "routers" with
> > 192.168.1.x addresses.
> >
> > I CAN also do the same with a 192.168.2.1 IF address on
> > eth0 and access ( web and ping ) say a 192.168.2.10.
> >
> > I CANNOT access Both a 1.x and 2.x boxes
> > ( say cheap routers ).
> > I wish to do so.
> > SO what do I need to do ?
> >
> > ??
> > ----------------
> > see below for various merged examples for readablilty . . . .
> >
> >
> > I'm aware that route and ifconfig can
> > assume certain parameters and auto-include them
> > when the user/script gives info: for example,
> > route can assume a netmask, or a -net option
> > in certain conditions.  It can also perform
> > an entry by setting an IP for an interface
> > such as eth0.
> >
> >
> > So below is an expanded version of efforts that
> > might be tried ( not all at one time, but one at a time
> > in various combos to try to understand in what order
> > route and ifconfig do things.
> >
> >
> >
> > [root@CentOS68 waltech]# route add -net 192.168.0.0 netmask 255.255.0.0
> eth0
> > [root@CentOS68 waltech]# route
> > Kernel IP routing table
> > Destination     Gateway         Genmask         Flags Metric Ref    Use
> Iface
> > 209.163.113.81  *               255.255.255.255 UH    0      0        0
> ppp0
> > 192.168.2.1     192.168.1.1     255.255.255.255 UGH   0      0        0
> eth0
> > 192.168.2.0     *               255.255.255.0   U     0      0        0
> eth0
> > 192.168.2.0     192.168.1.1     255.255.255.0   UG    0      0        0
> eth0
> > 192.168.1.0     *               255.255.255.0   U     0      0        0
> eth0
> > 192.168.1.0     192.168.1.1     255.255.255.0   UG    0      0        0
> eth0
> > 192.168.0.0     *               255.255.0.0     U     0      0        0
> eth0
> > default         *               0.0.0.0         U     0      0        0
> ppp0
> > [root@CentOS68 waltech]#
> >
> >
> > [root@CentOS68 waltech]# ifconfig
> > eth0      Link encap:Ethernet  HWaddr 00:0C:F1:8B:DF:79
> >           inet addr:192.168.1.1  Bcast:192.168.1.255  Mask:255.255.255.0
> >           inet6 addr: fe80::20c:f1ff:fe8b:df79/64 Scope:Link
> >           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
> >           RX packets:5558 errors:0 dropped:0 overruns:0 frame:0
> >           TX packets:4801 errors:0 dropped:0 overruns:0 carrier:0
> >           collisions:0 txqueuelen:1000
> >           RX bytes:3231777 (3.0 MiB)  TX bytes:490218 (478.7 KiB)
> >
> >
> > ppp0      Link encap:Point-to-Point Protocol
> >           inet addr:69.72.61.101  P-t-P:209.163.113.81
> Mask:255.255.255.255
> >           UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:552  Metric:1
> >           RX packets:7932 errors:4 dropped:0 overruns:0 frame:0
> >           TX packets:8563 errors:0 dropped:0 overruns:0 carrier:0
> >           collisions:0 txqueuelen:3
> >           RX bytes:2945795 (2.8 MiB)  TX bytes:693102 (676.8 KiB)
> >
> >
> > ----
> >  The government is lawless, not the press (people).
> >  ( [Supreme Court] Justice Douglas re: The Pentagon Papers )
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