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I am a junior admin and have been tasked with gaining an understanding of the network. I know and use some of the servers on the network, so am able to tracert/ping them to see the names/addresses of equipment there are along the way, and gradually build a map, but how do I put the feelers out to find out what's out there if I don't know the names of server etc?

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what operating system do you have on you computer. If you want some name of tools to use then it is easier for us to help you if we know what OS you have on your computer. – Raffael Luthiger Jan 8 '11 at 17:11
up vote 2 down vote accepted

Any time I want to map an unfamiliar network, I start with what the routing protocols can tell me. And usually the routing protocols can tell me pretty much everything. After all, the routing protocols have to know what the network looks like—and it's almost never exactly the way it's documented (if it's documented at all).

For an example of how this would go and to make things easy, let's say we're running OSPF. The great thing about OSPF (and link-state protocols generally) is that every router has already figured out the topology of the network. You just have to ask one of them:

> show ospf database router            

    OSPF link state database, area
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router  *      0x80000005  1083  0x2  0x532c  96
Router      0x80000003  1124  0x2  0xe1df  84
Router      0x80000004  1129  0x2  0xcf2f 108
Router      0x80000006  1133  0x2  0x83ed  60
Router     0x80000007   126  0x2  0xc1c9  84
Router     0x80000005  1086  0x2  0x847e  72
Router     0x80000004  1094  0x2  0x54da  60
Router     0x80000005  1148  0x2  0xd42c  60
Router     0x80000006   293  0x2  0xcf9   60

There we go, we have 9 routers in our OSPF area. Draw 9 boxes in visio and label them with those IPs. Next, to figure out how they're connected, ask for more detail:

> show ospf database router detail

    OSPF link state database, area
 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 
Router  *      0x80000005  1398  0x2  0x532c  96
  bits 0x0, link count 5
  id, data, Type PointToPoint (1)
  TOS count 0, TOS 0 metric 1
  id, data, Type Stub (3)
  TOS count 0, TOS 0 metric 1
  id, data, Type PointToPoint (1)
  TOS count 0, TOS 0 metric 1
  id, data, Type Stub (3)
  TOS count 0, TOS 0 metric 1
  id, data, Type Stub (3)
TOS count 0, TOS 0 metric 0

Router      0x80000003  1439  0x2  0xe1df  84

So here we're looking at the LSA for router It has 5 links that OSPF knows about. Two of the links id and id connect to other OSPF routers. Back to visio: draw a line between the box and the box. Draw another line between the box and the box.

The other 3 links are stubs, so OSPF will advertise the existence of those subnets, but there aren't any more OSPF routers on those links. You can just draw 3 clouds in visio labeled with the subnet information (plug in id and data for the network and mask for each) and draw a line from each to the router.

Keep doing that until you have a network map.

Of course it's never quite that simple, but no matter what, your routers have already figured out what your network looks like. Transcribing your routers' databases onto a map is always an enlightening experience. You'll learn lots about routing protocols AND your network.

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Thanks eater, exactly the kind of thing I was looking for! – Dave Jan 10 '11 at 18:24

Work with the "2nd line team." Being able to effectively work as a team with your coworkers is an invaluable skill; possibly moreso than being able to perform a network and server inventory. Also, finding and reading existing documentation is a better use of your time than figuring it all out from scratch. If no documentation exists, then start writing it as you go, and figure out how to publish (and publicize) it, because that will really make you stand out.

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I'd recommend trying in conjunction with nmap. Its a little tricky at first but if you do a broad enough scan you should be able to pick everything up.

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Thanks Jeffrey, I'll check it out. – Dave Jan 10 '11 at 18:25

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