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?
Answer
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 0.0.0.0
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *192.168.0.1 192.168.0.1 0x80000005 1083 0x2 0x532c 96
Router 192.168.2.1 192.168.2.1 0x80000003 1124 0x2 0xe1df 84
Router 192.168.5.1 192.168.5.1 0x80000004 1129 0x2 0xcf2f 108
Router 192.168.8.1 192.168.8.1 0x80000006 1133 0x2 0x83ed 60
Router 192.168.12.1 192.168.12.1 0x80000007 126 0x2 0xc1c9 84
Router 192.168.16.1 192.168.16.1 0x80000005 1086 0x2 0x847e 72
Router 192.168.20.1 192.168.20.1 0x80000004 1094 0x2 0x54da 60
Router 192.168.24.1 192.168.24.1 0x80000005 1148 0x2 0xd42c 60
Router 192.168.28.1 192.168.28.1 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 0.0.0.0
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *192.168.0.1 192.168.0.1 0x80000005 1398 0x2 0x532c 96
bits 0x0, link count 5
id 192.168.5.1, data 10.0.0.1, Type PointToPoint (1)
TOS count 0, TOS 0 metric 1
id 10.0.0.0, data 255.255.255.0, Type Stub (3)
TOS count 0, TOS 0 metric 1
id 192.168.2.1, data 10.0.1.2, Type PointToPoint (1)
TOS count 0, TOS 0 metric 1
id 10.0.1.0, data 255.255.255.0, Type Stub (3)
TOS count 0, TOS 0 metric 1
id 192.168.0.1, data 255.255.255.255, Type Stub (3)
TOS count 0, TOS 0 metric 0
Router 192.168.2.1 192.168.2.1 0x80000003 1439 0x2 0xe1df 84
...
So here we're looking at the LSA for router 192.168.0.1. It has 5 links that OSPF knows about. Two of the links id 192.168.5.1 and id 192.168.2.1 connect to other OSPF routers. Back to visio: draw a line between the 192.168.0.1 box and the 192.168.5.1 box. Draw another line between the 192.168.0.1 box and the 192.168.2.1 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 192.168.0.1 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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