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  1. Mr.Cheeks

    Mr.Cheeks 1st ever Gold Member! Gold Member

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    Question:

    A mate connects to the internet via Cable. His modem connects to his pc via USB.

    When i checked his ip address it was 82.44.xx.xx (his real ip), but under address type it said Assigned by DHCP (double click lan status box and select Support).

    I always thought that this would be a private address (192.168 address) as it was assigned by DHCP.

    I come to a conclusion but require confirmation if im correct.

    • The modem does not have DHCP "service" and submits the true address
    • As this is connected to USB and not via ethernet then DHCP does not work

    Also Windows LAN status box always says assigned by DHCP???

    Its a surf something modem (thin slim black one with 4 lights).
     
  2. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    It is the ISP's DHCP server that automatically assigns an IP address to the modem when it connects. The USB modem acts like a virtual NIC, the software/drivers do this.
     
    Certifications: C&G Electronics - MCSA (W2K) MCSE (W2K)
  3. AJ

    AJ Administrator Administrator

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    Just what Bluerinse said. You will find as well the IP address goes through a proxy server which may report you are using a different IP addy.
     
    Certifications: MCSE, MCSA (messaging), ITIL Foundation v3
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  4. r.h.lee

    r.h.lee Gigabyte Poster

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    Mr. Cheeks,

    According to RFC 2131, Section 1.3 - Problem definition and issues it states "DHCP is designed to supply DHCP clients with the configuration parameters defined in the Host Requirements RFCs. After obtaining parameters via DHCP, a DHCP client should be able to exchange packets with any other host in the Internet." It states that Appendix A specifies the "DHCP configurable parameters." Two of the listed parameters are "IP address" and "Subnet mask." Since I am able to post this reply from my private IP computer to a public IP server means there's IP to IP communication occuring. With that said, DHCP can configure the IP address of a DHCP client with any IP address, public or private, depending on the DHCP IP address range that is configured for the DHCP server. Therefore, DHCP is not limited to private IP addresses only.

    I hope this helps.

    Source:
    1. RFC 2131 - Dynamic Host Configuration Protocol - http://www.ietf.org/rfc/rfc2131.txt
     
    Certifications: MCSE, MCP+I, MCP, CCNA, A+
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  5. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    It is slightly more complicated than appears. Neither DSL nor Cable ISPs really use DHCP accross their links, but they *do* use PPP.

    Most simple modems - such as cable modems - are set to provide a very simple DHCP server that can only provide a single IP address. This address they extract from PPP.

    DHCP exists to provide IP addresses automaticaly, whether public or private. For example - the network I use at work has a DHCP system on it. It hands out public IP addresses.

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  6. Mr.Cheeks

    Mr.Cheeks 1st ever Gold Member! Gold Member

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    ...so if you sat behind a NAT then you definately would receive a private address?
     
  7. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    Usualy yes. There would be little point using NAT to translate to another public range. But it isn't an absolute requirement.

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+

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