关于linux的Bcast的疑问.请大家帮忙看看，谢谢啦

[root@localhost network-scripts]# cat ifcfg-Auto_em1
TYPE=Ethernet
BOOTPROTO=none
IPADDR=172.16.15.1
PREFIX=24
GATEWAY=172.16.15.254
DNS1=202.103.224.68
DEFROUTE=yes
IPV4_FAILURE_FATAL=yes
IPV6INIT=no
NAME="Auto em1"
UUID=03e6a739-ccde-40e9-88f9-1698565d5a4c
OnBOOT=yes
HWADDR=D4:AE:52:A3:65:6D
[root@localhost network-scripts]# ifconfig
em1 Link encap:Ethernet HWaddr D4:AE:52:A3:65:6D
inet addr:172.16.15.1 Bcast:172.16.255.255 Mask:255.255.255.0
inet6 addr: fe80::d6ae:52ff:fea3:656d/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:4257624 errors:0 dropped:0 overruns:0 frame:0
TX packets:6110407 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1029876738 (982.1 MiB) TX bytes:6390866849 (5.9 GiB)
Interrupt:35

em2 Link encap:Ethernet HWaddr D4:AE:52:A3:65:6E
inet addr:10.10.10.2 Bcast:10.255.255.255 Mask:255.255.255.0
inet6 addr: fe80::d6ae:52ff:fea3:656e/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:29030189 errors:0 dropped:0 overruns:0 frame:0
TX packets:31092427 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:4102474485 (3.8 GiB) TX bytes:3290376704 (3.0 GiB)
Interrupt:38


请问下网卡1的Bcast为什么不是172.16.15.255，而是172.16.255.255？如何调整才会是172.16.15.255。谢谢大家啦！