What is difference b/w Horizontal and Top down discovery ?

[SANJEEV4]

Please give a real-time scenario and provide any docs or sources to learn

[Tyson Elder]

I know this one is a bit old - but!

Horizontal - goes horizontally across a range/network. First IP: who are you?! Second IP: who are you?! and on and on until you get through the range. There is zero service context here.

Top down - Specifically starts with an entry point, think of that like a website URL http:www.website.com. Discovery goes: "Yo, http:www.website.com, which IP do you point to?"
Response: "Yo man, I point to 123.12.12.12 port 80"
MID: "Yo 123.12.12.12, who are you?"
Response: "Sup dude, I'm a load balancer and I forward to 124.14.14.14 on port 756"
MID: "Yo, 124.14.14.14, who are you?"
Response: "I'm a linux server my dear"
MID:  "Cool thanks, what do you have that listens on port 756?"
Response: "Uhhh, that's tomcat.exe and it uses this config file"
MID: "Yo yo, tomcat.exe config file, do you connect to anything?"
Response: "Yeah dude, I've got a reference in this config file that points to 125.15.15.15:1256"
MID: "heeyyyy 125.15.15.15, what are you and what do you have listening on port 1256?"
Response: "Man, you ask lots of questions, but I'm a Windows Server and I've got an Oracle Database on 1256, instance name WEBSITE DB"

Using all the above answers, we now have a specific service map, that tells us how www.website.com works and it's dependancies. This puts all of our infrastructure into a service context. We now know if a certain server goes down, what is going to be impacted.

I compare it to Horizontal Discovery finds all your Lego, but does not click it together.
Top-Down discovery takes that room full of lego all over the floor, and clicks it together which shows you your services.

[Pratiksha]

Horizontal discovery helps you scan the infrastructure and store in cmdb it is not concern about how two CI may be related. 

Top Down discovery done after horizontal discovery . It relates two CI. For example a web service individually can hosting server, load balancer, data base and so on. All of them are discovered with the Horizontal discovery. Now you want to know for a particular service what all CI's has been used and how they are related, we use top down mapping. 

If you want to start then a now learning course for discovery you can do. It will give you basic idea. Also I would recommend to check for now learning CMDB course. Once both are done you can go with service mapping.  Best of luck.