Re: Point-to-Point links - route IP Block with DSL

You should know up front that I've read about but never used PPPoE or
DHCP. Hopefully any gross mis-statement in my reply will be corrected
by others more capable. (I'd hoped someone else with hands-on experience
would reply but nobody has volunteered.)

g18c@xxxxxxxxxxx wrote:
I've been given a /29 ip block from my ISP for use over DSL. I am a bit
confused however with point to point links.

My DSL connection is working properly and I have set it to receive a
dynamic address from DHCP. It so happens dsl0 always gets allocated one
of my block ip addresses, this is where I start to get confused.

Say for arguments sake my addresses are as follows (these are example
addresses): -
Subnet mask:
Network address is therefore, and broadcast

When I type 'ip addr' I see my DSL connection has been assigned one of
the block ip addresses,, and its peer is

6: dsl0: <POINTOPOINT,MULTICAST,NOARP,UP> mtu 1492 qdisc pfifo_fast
qlen 3
inet peer scope global dsl0

My example route table is as follows:

Destination Gateway Genmask Flags Iface UH dsl0 U eth0 U eth0 U lo UG dsl0

a) Is the reason that point-to-point links do not have to be on the
same subnet because as the mask is there is *no
subnet*? I can see from above that if the route table fails to find
specific route for data it chucks it out dsl0 to be handled by the
gateway, i presume that is correct?

Yes, it is a host route, with no subnet.

Mostly correct, the PPP interface (dsl0) gets all traffic from your
host - which is not otherwise locally routed - via the default route
(UG above) and it is then sent to the gateway.

b) I can't see an entry for the dsl0 address it was assigned, i.e., yet when I type 'ping' there is no delay
and it replies straight away. I thought the route table dealt with
everything, how come I can ping when there is no entry
for it? is the local IP address of your host's PPP interface.
You should be able to see the local as well as the remote IP address
with "ifconfig dsl0" (the renaming of the local PPP interface pppX
to dslX is relatively new so no guarantee). Traffic to your host
from your host is handled locally by loopback. "tcpdump -vn -i lo"
will show the pings. This will also be true for Ethernet interfaces
configured with an IP address but with all routing associated with
that interface removed.

c) I will add an additional card eth1 and have that as the public ip
segment using - . Since my dsl0 is using, and I have the default route as, can I set my
eth1 to (and add a route for out eth1)?
Can the machine have interfaces which have the same addresses as
point-to-point links?

Don't you mean configure eth1 for with broadcast, assuming configuration is done with ip? I think that
will generate an interface for eth1 with IP address and
a /29 network subnet starting at

Yes, the connection host can have a LAN interface that has the same
IP address as the local address of the PPP interface. Since you
are have a fixed subnet of routable IP addresses I suspect it is the
other hosts on the LAN that get their IP addresses with DHCP from the
connection host while the connection host itself obtains it's PPP IP
address ( by PPP negotiation. If this is so then eth1
should also have that IP address so the address can be used as the
default gateway for the other LAN hosts.

d) Is routing different for ppp links than normal subnets? How can
there be two addresses, the local and remote address, when the mask is
/32 for my ppp link?

A little different. A PPP link is between two hosts and the PPP
interface on each is provided with both the local and the remote IP
address, which are usually determined by PPP negotiation. By default
only a host route (/32) to the remote is set on the local host when
the PPP interface comes up. When an Ethernet interface comes up it's
network route is set.

I have tried searching for examples on the internet but couldn't find
any ones which covered my specific questions on ppp links as above. If
any responses could include links or short explanations I would
appreciate it very very much!

Clifford Kite Email: "echo xvgr_yvahk-ccc@xxxxxxx|rot13"
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