I believe there is some confusion....may be what you're calling "broadcast" is actually "gateway" ?
First check how your router is set (basically if it is acting as DHCP server. Most of routers usually do as default).
If this is the case just use
/sbin/dhcpcd -d -t 15
where the -d option stands for daemon and -t 15 is the timeout setting.
If your router is configured not to act as DHCP server use
/sbin/ifconfig eth0 192.168.x.x netmask 255.255.255.0
the broadcast address will be automatically guessed by ifconfig (192.168.x.255)
To add your gateway do
/sbin/route add default gw 192.168.x.y
where 192.168.x.y is the address of your router.
Slackware comes with a very useful tool called "netconfig". This tool will make your life easier.
On /etc/resolv.conf you must add the address of the DNS servers of your IP provider. Instead you could use the addresses you find here
There are some extra small benefits
Hope this helps