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10515 Views 6 Replies Latest reply: Jun 20, 2010 3:53 PM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

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How to determine the subnet mask of a public IP address?

Jun 19, 2010 7:15 PM

ciscohardhat 48 posts since
May 24, 2010

After determining the public IP address of a website via a DNS request, how can I further determine the subnet mask of the IP address?

  • Angela 746 posts since
    Jan 29, 2010

    Hi,

     

      I would like to know why do you need to know the subnet mask of a public IP address? Is it needed for ARP? I don't think so. Is it needed for any other protocol (except routing protocol)? I don't think so either. Also, because public IP address are ALWAYS unique, why do you really need the subnet mask for?

     

      I can only think of subnet mask when I need to assign IP addresses (static or via DHCP) and using 'network' statement.

     

     

    Regards

     

    Message was edited by: Angela

  • Angela 746 posts since
    Jan 29, 2010

    Being curious is certainly good, knowing more can't hurt. However, I don't think there is a way excepting calling your ISP to find out about it, cuz the Internet routing table can contain just about anything and people love VLSM these days. Still, I don't think there is any reason to look for the subnet mask.

     

    Regards

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009

    An ICMP type 17 message, is a mask request.   An ICMP type 18 is an ICMP mask reply.

     

    IOS, by default, will not respond to one of these requests.

     

    Best wishes,

     

    Keith

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008

    You could do a "whois" on the IP address, but that will just tell you the mask allocated to the company, which is not necessarily equal to the mask used on whatever subnet(s) they created with them!

     

    Scott

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