Enterprise Manager :: Why Scan Address Used In OEM

I am an OEM Gird Control 11g server with Oracle 11.2.0 running on OEL 5.3

I also have 2 node RAC cluster running Oracle 11.2.0 on OEL 5.3. I have installed OEM Agent on nodes of this cluster. I can see them on OEM Grid Console. When I check the configuration of the Database Instance GTRD1 (one of my target instance)I can see that the "Listener Machine Name" is the management IP address of target database server. Similarly Database Instance GTRD2 has the management IP address of target database server. When I click test conenctions it works. This is without entering any reference to this server/IP address in /etc/hosts file.

But when I select targets->all targets->Database Instance GTRD1 and then try to access performance tab, it does not connect. I get this error "The Network Adapter could not establish the connection"

Looking at the "emoms.trc" I find the follwing

2012-04-30 14:02:12,071 [EMUI_14_02_12_/console/database/instance/sitemap] ERROR perf.sitemapPerfChart logp.251 - java.sql.SQLException: The Network Adapter could not establish the connection

The Connect Descriptor was (description=(address=(host=GTRD-scan.GTRD)(protocol=tcp)(port=1521))(connect_data=(service_name=GTRD1)(instance_name=GTRD1)(UR=A)))java.sql.SQLException: The Network Adapter could not establish the connection

The Connect Descriptor was (description=(address=(host=GTRD-scan.GTRD)(protocol=tcp)(port=1521))(connect_data=(service_name=GTRD1)(instance_name=GTRD1)(UR=A)))

But when I enter the scan address in the /etc/hosts as "112.5.14.154 GTRD-scan.GTRD" Then everythign work works.
NOTE: 112.5.14.154 is GTRD1 server and 112.5.14.155 is GTRD2 server

Now my questions are:
1. Do I must enter scan address/ref "GTRD-scan.GTRD" in /etc/hosts?
2. Is there are way to change it to use hostname or the IP address?
3. If I am to use scan address, then how I refer to the other IP address in hosts file?