I am wondering what, if anything, this content of the /etc/iproute2/rt_realms file mean?
# # reserved values # 0 cosmos # # local # #1 inr.ac #2 inr.ruhep #3 freenet #4 radio-msu #5 russia #6 internet
Realms in iproute2 are a way of clustering sets of routes into groups. Packets following each route will be considered part of the corresponding realm, this classification can then be used to do bandwidth throttling, apply filters (e.g. with iptables), track usage statistics, and more, realm-wise.
Realm IDs are integers, but can also be called by name if configured. The file you mention (/etc/iproute2/rt_realms) is where you link the names to the numerical value of the realm. The contents as you show them are just examples of how the file can be constructed (all lines are commented with #).
You can find more information regarding realms in iproute2 here: http://www.policyrouting.org/iproute2.doc.html#ss9.9