Can anyone explain this code to me?

Is there anyone .. who can explain these nids rules of snort.. just one line discription is enough..code mean?

• (a): var HOME_NET 203.27.40.0/24 var EXTERNAL_NET !203.27.40.0/24 (Discription here that what the above code means ? thanks) (b): alert (Discription here) (c): tcp $EXTERNAL_NET any -> $HOME_NET 993 (Discription here) (d): (msg:"IMAP SSLv3 invalid Client_Hello attempt"; (Discription here) (e): content: "HELLO"; nocase; depth:5; offset:3; (Discription here) (f): reference: cve,2004-0120; reference: nessus,12204; (Discription here) (g): classtype: attempted-dos; (Discription here) (h): sid: 25310; rev: 6;) (Discription here)

• Answer:

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