ping - Junior admin - how to discover/map the network to increase understanding?

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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?

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.