StartupResult RakPeer::Startup( unsigned short maxConnections, SocketDescriptor *socketDescriptors, unsigned socketDescriptorCount, int threadPriority );

The first thing you should do is call RakPeerInterface::Startup(). Startup() will

1. Generate RakNetGUID, a unique identifier identifying this instance of RakPeerInterface. You can get it with RakNetGUID g = rakPeer->GetGuidFromSystemAddress(UNASSIGNED_SYSTEM_ADDRESS);
2. Allocate an array of reliable connection slots, defined by the maxConnections parameter. This may be the maximum number of players for your game, or you may want to allocate extra to keep some buffer slots open, and manually control who enters your game or not.
3. Create 1 or more sockets, described by the socketDescriptors parameter

Before calling Startup(), generally only raw UDP functions are available, including Ping(), AdvertiseSystem() and SendOutOfBand().

maxConnections

RakNet preallocates most memory used for connecting to other systems. Specify maxConnections to be the maximum supported number of connections (both incoming and outgoing) between this instance of RakPeerInterface and another instance. Note that if you want other systems to connect to you, you also need to call SetMaximumIncomingConnections() with a value lower than or equal to maxConnections.

socketDescriptors parameter

In 95% of the cases, you can pass something like:

SocketDescriptor(MY_LOCAL_PORT, 0);

For MY_LOCAL_PORT, if running a server or peer, you must set this to whatever port you want the server or peer to run on. This is the remotePort parameter you will pass to Connect(). If running a client, you can set it if you want to, or use 0 to automatically pick a free port. Note that on Linux you need admin rights to use a value lower than or equal to 1000. Some ports are also reserved, although there is nothing stopping you from using them. See http://www.iana.org/assignments/port-numbers

If desired, you can also create an array of socket descriptors:

This is for advanced users that want to bind multiple network cards. For example one card to go to the secure server behind the LAN, and another to the internet. To access the different bindings, you would pass the index of the binding to use to functions in RakPeerInterface that have a connectionSocketIndex parameter.

IPV6 is the newer internet protocol. Instead of addresses such as 127.0.0.1, you may have an address such as fe80::7c:31f7:fec4:27de%14. Encoding takes 16 bytes instead of 4, so IPV6 is less efficient for games. On the positive side, NAT Punchthrough is not needed and should not be used with IPV6 because there are enough addresses that routers do not need to create address mappings.

IPV6 is disabled by default. To enable IPV6 support, set the socket family to AF_INET6. For example:
socketDescriptor.socketFamily=AF_INET6;

IPV6 sockets can only connect to other IPV6 sockets. Similarly, IPV4 (The default) can only connect to other IPV4 sockets.

threadPriority

For Windows, this is the priority of the RakPeer update thread passed to _beginthreadex(). For Linux, it is passed to pthread_attr_setschedparam() for pthread_create(). The default parameter is -99999, which on Windows means use priority 0 (NORMAL_PRIORITY) and on Linux means use priority 1000. For Windows, the default parameter is fine. For Linux, you'll want to set this to be whatever value you normally use for normal priority threads.