Java Reference
In-Depth Information
much more commonly used behind the firewall within a single organization than across
the global Internet.
As far as the application itself, you need to pay attention to an additional header field
in the datagrams called the Time-To-Live (TTL) value. The TTL is the maximum num‐
ber of routers that the datagram is allowed to cross. Once the packet has crossed that
many routers, it is discarded. Multicasting uses the TTL as an ad hoc way to limit how
far a packet can travel. For example, you don't want packets for a friendly on-campus
game of Dogfight reaching routers on the other side of the world.
Figure 13-2
shows
how TTLs limit a packet's spread.
Figure 13-2. Coverage of a packet with a TTL of five
Multicast Addresses and Groups
A
multicast address
is the shared address of a group of hosts called a
multicast group
.
We'll talk about the address first. IPv4 multicast addresses are IP addresses in the CIDR
group 224.0.0.0/4 (i.e., they range from 224.0.0.0 to 239.255.255.255). All addresses in
this range have the binary digits 1110 as their first four bits. IPv6 multicast addresses
are in the CIDR group ff00::/8 (i.e., they all start with the byte 0xFF, or 11111111 in
binary).
