Networking

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Networking
Why is it useful to have both a logical design and a physical design of a network?

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Physical design – cable lengths, switch closets, that mini router behind a desk to ‘temporarily’ connect the new guy. Failure to document the layout as you build it will double the cost of maintenance and growth.
Logical – imagine trying to explain to your boss why accounting is no longer connecting to the server, because you don’t remember the VLAN setups. If you only have fifty users and one subnet you can keep it in your head, but don’t take any time off. Documentation is how you get the money to cover growth. If you can’t show what you have and how it is being used, why expect to get money when they want more connections next year.

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  • Bobkberg
    I liked Howard's answer (we seem to think similarly in an IT vein), but since he took the first good answer, he's thrown down the proverbial glove...so now it's my turn. :-) Another reason for physical diagrams is keeping track of how much rack space is left, the ability to "see" heat loads, reminding you of what physical box can fit where, and other real-life physical constraints that might affect a project, operation or whatever. It's really tough on installation day to discover that there's a 12" reinforced concrete wall (or other unwieldy obstacle) in your way, or that removing a certain wall will cause several tons of air-conditioning equipment to fall on you (I've SEEN that one - before it was too late). Physical diagrams make you go out and LOOK at the situation to keep them current - which can prevent running into these obstacles before it's too late to change plans. Logical Documentation is necessary because of the "out of sight, out of mind" situation. If you don't deal with a particular area or group regularly, chances are that you won't remember the critical details that could affect some other project. Having a document to track these things is invaluable. Bob
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  • Khilving
    The two previous replies are both good, but let me take it a step further. You really should have a logical, a physical, and a component design. The logical defines how the network works and covers such things as protocols used, redundancy plans, addressing schemes, and supporting elements like DNS and NAT. Include a why for each aspect, link it to business requirements, and you will be able to change as requirements change. The physical is the where, for the obvious implementation and support aspects already covered, The component is the what. Vendor, model, serial number. Here again, the why is a useful piece of information. Perhaps a particular make or model was the only option for meeting the logical design requirements when first selected. Knowing that is useful for determining an acceptable second source, or even changing vendors completely. Often such changes are desired becuase of new business relationships outside of any IT related functions, or when a merger occurs.
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  • ggiuliano
    Hi In reply to all those that took time out to explain the benefits of having Logical, Physical and Component diagrams, do you know of any websites that have examples to illustrate how to Layout/Document these...??? Cheers
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  • Jjrcol
    Thank you everyone for your replies....they really helped a lot. In regards to the last reply, no, I do not know any specific sites to view the layouts, I would have to search. Thank you all again.. jjrcol
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  • Hedgehog
    It is obvious that Howard, Bob and khilving have done many times this inventory of physical & logical designs as well as components. Would you mind explaining how you keep track of changes? Do you use a simple Excel sheet, an actual database or some bespoke software? Cheers H.
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  • DrillO
    Wow.....I was really late getting to this one. As Bob so often observes, all the good answers are gone.....Bravo guys. I would like to make an ovservation or two if I may. One of the tuffest thigs to get (and it has been the case for many years) is good documentation of anything "tech". So many guys I know and have worked with just to get the instal done and get out. Very few contracts involve a requirement for documentation ....sad really. I document everythng, but that is only half of the game. As was mentioned, maintenance of your documentation is also critical. What happens if changes are made, and they always are? I even go so far as to copy blueprints and make all sorts of supporting notes and diagrams on them and keep them with the other documentation so that if I am ever hit by a bus, the new guy should be able to step in very easily. Even when I am away, there are sets of proceedures and documentation "light" available in case my "second" has to call someone in to assist. The comments about budgeting and funding is also very valid.....all too often our hands are tied by people who don't understand. If the documentation is correct and current, it is much easier to build the case. Hope that helps. Paul
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  • Bobkberg
    I can speak to two of the forms, maybe 2.4 of them. The logical diagram is usually done in Visio showing logical and physical paths, IP (or other) addressing, components (obviously not to scale), and sometimes other information. This is done in layers (a concept that Visio supports) so that you can hide some of the information (like component descriptions) so that they don't clutter up the drawing, but yet know that it's available. To echo Paul/DrillO as well maintenance is critical. It often falls on either the junior person in the group, or is shared by all with a change control system to keep older records available. The geographical data can be kept in a large multi-page spreadsheet. Excel handles this by letting you define a hyperlink within the current document on (for example) Sheet1, which points to Sheet2, where the Chicago data is stored. Then you can change the label for that cell to identify it. If you use this method, then most of your sites can be described and seen on one page - with all of the different site's detail readily available in the same document. While answering your question, it occurred to me to wonder what OTHER spreadsheets might be out there. Here's a link http://www.j-walk.com/ss/other/ Bob
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  • Guardian
    True all documentations has to be updated. With physical and logical, it assists when planning for expansion and upgrades, you know which hardware is where, and software used. which areas suit which hardware components and the users that use the IT resources. If you're upgrding the OS platform you will need to know all this the hardware compatibilities and software requirements with what you actually have on the network. Whilst you must keep a steady inventory of the entire system, hardware, software and get deep into everything: Bios version (disk for that), OS, RAM (and type), system board type and all etc... And keep it regulary updated (at least after every 4 to six months) Regards Newton PS: try also techrepublic
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  • Bobkberg
    I just love it when we all come together and tackle a problem from all 46 sides. (Just a joke). Seriously - I learn something from every one of these multi-respondent questions - the more the better. Bob
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  • NKOSANA
    because physical & logical designs helps in mananging a companies resource layout so that they meet the companies logical structure
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