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my public IP is 89.75.42.70. Why RIP packet contains such information? I don't understand the IP notation and what it means.

IP Address: 89.75.40.0, Metric: 1

Address Family: IP (2)

IP Address: 89.75.40.0 (89.75.40.0)

Metric: 1

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up vote 3 down vote accepted

RIP is Routing Information Protocol, and is a very basic (and early) protocol for announcing routes available on a given interface. It's a broadcast protocol. For RIP packets that do NOT contain a Netmask, like yours, it presumes the use of the old and very deprecated classful designation of Subnets. In your case, it is announcing a route for 89.75.40.0/24, the old, deprecated, Class C.

Specifically, RFC1058 section 3.2 defines how these no-subnet announcements work:

When a host evaluates information that it receives via RIP, its interpretation of an address depends upon whether it knows the subnet mask that applies to the net. If so, then it is possible to determine the meaning of the address. For example, consider net 128.6. It has a subnet mask of 255.255.255.0. Thus 128.6.0.0 is a network number, 128.6.4.0 is a subnet number, and 128.6.4.1 is a host address. If there is a non-zero host part, there is no clear way to determine whether the address represents a subnet number or a host address.

[Emphasis is mine]

RIPv1 had no field for subnet-mask. RIPv2 has one, but still allows absent subnet-mask.

In your case where network 89/8 was in the old Class A range, this suggests that your RIP packet is actually a v2 packet. V2 speaks CIDR, it's just optional, so the HOSTS getting the RIP announcement would evaluate it versus whatever subnet-mask they have on their own network configuration.

That address is out-of-range, so it would be seen as an announcement that the specified network, wherever it is, can be reached at the IP address of the source. As the announced address ends in Zero, and no subnet-mask was given, a host may assume that the netmask is /24.

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so 89.75.40.0 is another network, not my network? it just says that this network is in my scope? what exactly this means? I have checked this is the same ISP –  bits_international Jul 21 '13 at 16:00
    
how do you know it is/24 network? also doesn't C network contain addresses in range 192.0.0.0-223.255.255.254 ? en.wikipedia.org/wiki/Classful_network –  bits_international Jul 21 '13 at 16:16
    
@restart.localhost.localdomain It can tell from the number of zeroes at the end of the announced network. .0 means, class C. 0.0, means Class B. The announcement you're seeing is an announcement that the stated network is available behind the IP address of the device that issued the RIP packet. –  sysadmin1138 Jul 21 '13 at 16:24
    
but how this can be not in conflict with fact that class C contains addresses in range 192.0.0.0 - 223.255.255.254? 89.xx.xx.xx doesn't fit into it, does it? –  bits_international Jul 21 '13 at 16:30
1  
@restart.localhost.localdomain It could also be a v2 packet, without a listed subnet. Subnet is not a required field for the v2 protocol. –  sysadmin1138 Jul 21 '13 at 19:43
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