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  1. Senior Member
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    #1

    Question 0.0.0.0/8 and 0.0.0.0/32 blackhole routes

    I have seen in router configuration examples following null routes:

    Code:
    ip route 0.0.0.0 255.0.0.0 Null0 250
    ip route 0.0.0.0 255.255.255.255 Null0 250
    How should one understand such routes? First one blackholes all those packets which have destination IP address most-significant byte zero? This is a security measure as such network does not exist? And second static route blackholes all those packets which have destination IP address 0.0.0.0?
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  3. Senior Member
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    CCNP, CCNA
    #2
    theory would say 0.0.0.0 255.0.0.0 matches 1.0.0.0 2.0.0.0 and so on
    whereas 0.0.0.0 255.255.255.255 matches 0.0.0.0 as you said
    nothing like firing up a router and do some tests I will try labbing this!
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  4. Senior Member SteveO86's Avatar
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    #3
    0 /8

    0.0.0.0/8 - Addresses in this block refer to source hosts on "this" network. Address 0.0.0.0/32 may be used as a source address for this host on this network; other addresses within 0.0.0.0/8 may be used to refer to specified hosts on this network [RFC1700, page 4].

    From RFC 5735It's for security. It drops packets that the router should not forward.It's like denying RFC 1918 addresses your internet facing interfaces, since RFC 1918 addresses should not be coming from the internet. (RFC 1918 are non publicly routable IP's on internet)
    Last edited by SteveO86; 02-12-2012 at 05:02 AM.
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