How do I permanently redirect subdomain to specific URL?

Hi, I’m new to Digital Ocean, and also to nginx.

I have a droplet hosting a couple of different sites, and most traffic for a domain—let’s call it domain.com—should be hosted here. But sub.domain.com needs to redirect to a specific URL, e.g. 12.34.56.78/a/particular/path/.

It seems like DNS only directs to servers, and so I need to create a subdomain-specific redirect.

My file in /etc/nginx/sites-available looks like this:

server {
  server_name domain.com;

  root /var/www/domain/html;
  access_log /var/log/nginx/www.demo.com.access.log;
  error_log /var/log/nginx/www.demo.com.error.log;

  # INCLUDE CONFIG INFO COMMON TO ALL SITES
  include global/common.conf;

  # INCLUDE COMMON WORDPRESS INFO
  include global/wordpress.conf;

  include fastcgi_params;
  fastcgi_param SERVER_NAME $host;
  fastcgi_param DOCUMENT_ROOT root;
}

server {
  server_name  sub.domain.com;
  rewrite ^(.*) http://12.34.56.78/a/particular/path/ permanent;
}

This does not seem to be working, and my browser returns an ERR_NAME_NOT_RESOLVED.

How do I redirect that subdomain traffic to that external address? Note: I want the address in the browser to still say ‘sub.domain.com’.

Thanks anybody for your help!