Point to point protocol in ifconfig output

[Sunny Jaisinghani]

Hello All,

I want to know what is the significance of POINTOPOINT protocol

lan0:1: flags=853
inet 172.16.101.159 --> 255.255.252.0 netmask ffff0000 broadcast 172.16.255.255

lan0: flags=843
inet 172.16.101.85 netmask ffff0000 broadcast 172.16.255.255


The virtual interface is used for some service. It works well.

What will happen if i configure lan0:1 as

Or to reframe my question

What is difference between

ifconfig lan0:1 172.16.101.159 netmask 255.255.252.0 up

ifconfig lan0:1 172.16.101.159 255.255.252.0 up

Thanks
Sunny

[Marco A.]

Hi,

There's no difference between them, basically the syntax requires the "netmask" when configuring , but in fact is the same ....for functionality ... there's no difference.

Regards,

Marco,

[Sunny Jaisinghani]

Even if i do not give "netmask" in the command the interface comes up and shows in ifconfig output.

From what i have read and understood it seems if i run the command

ifconfig lan0:1 172.16.101.159 255.255.252.0 up

The output of ifconfig lan0:1 would be

lan0:1: flags=853
inet 172.16.101.159 --> 255.255.252.0 netmask ffff0000 broadcast 172.16.255.255


and if i use the command

ifconfig lan0:1 172.16.101.159 netmask 255.255.252.0 up

the output would be

lan0:1: flags=853
inet 172.16.101.159 netmask ffff0000 broadcast 172.16.255.255

will both provide me the same functionality or the POINTOPOINT will make some differences.

Thanks
Sunny

[VK2COT · Accepted solution]

Hello,

Coincidentaly, my students made the exact same
error last week when I was teaching HP-UX
Security course.

a) The first command is a regular option
to set a "normal" network interface:

ifconfig lan0:1 172.16.101.159 netmask 255.255.252.0 up

b) The second command sets link
between two hosts (172.16.101.159 and
255.255.252.0):

ifconfig lan0:1 172.16.101.159 255.255.252.0 up

This option is used for point-to-point IP links that involve only two hosts. This option is needed to configure SLIP, PLIP, PPP interfaces, for example.

Interface setup contains a union for either
the broadcast address associated with an
interface on a network that supports broadcasts (for example, Ethernet), or the
destination address associated with an
interface that reaches only one other place.

Broadcast networks have some topological
restrictions. For example, you can not have
two Ethernet interfaces with addresses on
the same network or subnet. Routing decisions
on broadcast networks examine the interface's
address and subnet mask to know whether a
packet's destination is reachable via ARP
or routing.

There is no reason to use ARP on a
point-to-point link, and no need for a
broadcast address because there is only one
other system on the link.

Instead, interface contains the address
through which packets may be routed.
There is no topological restriction against
duplicated local addresses. It means you can
have many point-to-point links, each
with the same local address, distinguished
in the routing table by their differing
destination addresses.

Cheers,

VK2COT

[Sunny Jaisinghani]

Hi

Thanks for this valuable information.

However, i need to to know if my interface will work fine when i configure it in both ways

i.e

a) The first command is a regular option
to set a "normal" network interface:

ifconfig lan0:1 172.16.101.159 netmask 255.255.252.0 up

b) The second command sets link
between two hosts (172.16.101.159 and
255.255.252.0):

ifconfig lan0:1 172.16.101.159 255.255.252.0 up


I cannot make any changes to the interface as it is a production server.

Thanks
Sunny

[VK2COT]

Hello,

No, option b) will not work for you:

b) The second command sets link
between two hosts (172.16.101.159 and
255.255.252.0):

ifconfig lan0:1 172.16.101.159 255.255.252.0 up

According to RFCs, the first octet
of IP address 255.255.252.0 is 255 (Class E).
This is a class meant for experimental
purpose only. The first octet in Class E
is between 240 to 255. (Starts with binary
bits 1111). The Class E is also a special
purpose reserved class, and addresses in
this range are not assigned as IP addresses
on an IP network, including Internet.

Just as a reminder, you all know about
"traditional" classes: A, B, C.
There is also Class D:

Class D - This is a class meant for
multicasting only, for sending multicast
messages to other groups of host machines.
First Octet - The first octet is between 224
to 239. (Starts with binary bits 1110).
The class D is a special purpose reserved
class, and addresses in this range are not
assigned as IP addresses on an IP network,
including Internet.

Cheers,

VK2COT

[Sunny Jaisinghani]

But i already have an interface configured using command

ifconfig lan0:1 172.16.101.159 255.255.252.0 up

The ifconfig lan0:1 shows

lan0:1: flags=853
inet 172.16.101.159 --> 255.255.252.0 netmask ffff0000 broadcast 172.16.255.255

I want to know if i use the normal command

ifconfig lan0:1 172.16.101.159 netmask 255.255.252.0 up

will it give me the same result.

Sunny

[Matti_Kurkela]

Impossible to know without a lot more details of your network setup. Is lan0:1 being used as a regular IP Alias, or is it supposed to do something special? Is it used as an entry point for incoming connections, an exit point for outgoing connections, or both?

Your best way to solve this problem would probably be to explain your it to your network administrator and/or someone who has a detailed understanding of what this system is supposed to do with the lan0:1. They should be able to verify that the parameters of the normal-style command are correct.

My *guess* would be that the use of the normal command *probably* will not harm anything, and *may* be exactly correct. However, I don't know enough about your system and network to say for sure.

In theory, your current configuration should not work at all. If it works, it probably means that the lower levels of the network driver stack don't really understand what is being requested, and just ignore the error and work as they normally do. Or maybe some workaround has been applied on the other host(s) that communicate with the IP address 172.16.101.159. (Something like hard-coding the MAC <-> IP address mapping using the "arp" command, maybe?)

I think the behaviour of your current non-standard configuration is completely undefined and may change if any component in the system is changed (OS version and patch level, NIC model, network switch model and firmware, the OS versions and patch levels of the other hosts communicating with this one...). So it would be highly desirable to use the normal configuration.

MK

[Sunny Jaisinghani]

Hi,

This interface is used for an alias for print server .

This interface is an entry point for print requests from oracle application servers and an exit point for outgoing requests sent to the n/w and remote printers.

I think it is better to consult the network administrators and ask them to decide what is better and why the configuration
is the way it is.

Thanks for the information

Sunny