you can write it once. unfortunately there's no getty escape sequence to show the ip address of eth0 but there's an escape sequence you can use in /etc/issue to show the hostname: \n
you can always statically set the ip of the machine in /etc/issue by writing it directly into the file. beware that the file /etc/issue.net is used for remote logins so you may want to edit that as well.
anyway, after working in the terminal window you will loose the initial screen and the ip/host address information with it. best way is to set the hostname in the prompt (most linux distros do) using \h or \H or again, statically setting the ip address you know is set on the host in the PS1 variable
you can set PS1 variable in /etc/profile on most linux distros.