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A: ODDNS is primarily designed to bypass the DNS root zone and the TLD registries (such as those for ".com" and ".org"). Instead of using those, each user is expected to maintain a database of (second-level) domains (like "gnu.org") and the IP addresses of the respective name servers. Resolution will fail if the target name servers change IPs.
A: In GNUnet you set up a node (a peer). It is identified by an ID (hash of its public key) and has a number of addresses it is reachable by (may have no addresses, for instance when it's behind a NAT). You specify bandwidth limits (how much traffic GNUnet is allowed to consume) and datastore quote (how large your on-disk block storage is) . Your node will then proceed to connect to other nodes, becoming part of the network.