When you boot your Raspberry Pi with Raspbian, it will get its ip address from a DHCP server. Most tutorials I’ve seen, ask you to login to a ‘router’ (that serves as DHCP server as well) to find the assigned ip address. When you haven’t access to the DHCP server, how do you find the Pi’s ip address?
You should know the MAC address of a Raspberry Pi always has a ‘b8:27:eb’ prefix. Using a Linux utility called ‘arp-scan‘ you can discover MAC addresses and their corresponding ip addresses. Arp-scan sends ARP packets to hosts on the local network and displays any responses that are received.
First, install arp-scan:
apt-get install arp-scan
Then locate all Raspberry Pi’s on the local network using this command:
arp-scan --interface=eth0 --localnet | grep b8:27:eb
You will need to be root, or arp-scan must be SUID root, in order to run arp-scan, because the functions that it uses to read and write packets require root privilege.
Result:
172.16.12.34 b8:27:eb:00:11:ab (Unknown)
Using the ip address, it is now possible to SSH into your Raspberry Pi:
ssh -l pi 172.16.12.34
Remi Bergsma's blog 
When I try to install it I get the message:
-bash: apt-get: command not found
Any ideas?
you forgot to use “sudo”
sudo arp-scan –interface=eth0 –localnet | grep b8:27:eb