When creating an Internet emulator, we need to assign IP prefixes to networks and assign IP addresses to hosts. There are two methods to do this:
Inside the emulator, when we create a network or node (host or router), default network prefix and IP address will be assigned to them. For networks inside an autonomous system or an Internet exchange, the default network prefix assignment uses the following scheme:
10.{asn}.{id}.0/24
The asn is the autonomous system number, and {id} is the nth network created.
For example, for AS150, the first network is 10.150.0.0/24, and the second one
is 10.150.1.0/24. For Internet exchanges, the {id} part is always 0.
For example, the default prefix of IX100 is 10.100.0.0/24.
When a node is added to a network, the IP address for the host
is assigned with AddressAssignmentConstraint.
The default constraints are as follow:
For example, in AS-150, if a host node joins a local network, its IP address
will be 10.150.0.71; the next host joining the network will have IP address
10.150.0.72. If a router joins a local network, its IP address will be
10.150.0.254, and if the router joins an internet exchange network (say
IX100), its IP will be 10.100.0.150.
If the autonomous system number is greater than 255, the default network prefix assignment will not work. We need to explicitly set the network prefix when creating a network. See the following examples:
# For the network in Internet exchanges
base.createInternetExchange(asn = 33108, prefix = '206.81.80.0/23')
# For the network in autonomous system
as350 = base.createAutonomousSystem(350)
as350.createNetwork(name='net0', prefix = '128.230.0.0/16')
The automatic IP address assignment for hosts does not work in certain situations. For example, if the ASN is 350, when its BGP router joins a network in an Internet exchange (say IX100), it cannot use the automatic assignment, because according to the assignment scheme, the IP address would be 10.100.0.350, which is not a valid IP address. The joinNetwork API does take an address argument, allowing users to explicitly provide an IP address:
as350.createRouter('router0').joinNetwork('net0').joinNetwork(netname='ix100', address='10.100.0.35')
The actual automatic assignment scheme is implemented in the AddressAssignmentConstraint class. If you want to implement your own automatic assignment scheme, you can derive from this class (i.e., implement a subclass), and then implement your own assignment scheme inside the class. Both createInternetExchange and createNetwork accept the aac argument, which takes an instance of the AddressAssignmentConstraint class. By default, aac is set to None, so the default implementation class is used.