Multi SSH Key pairs on one client

If there exist multi SSH key pairs and each pair works on a SSH connection, then we need to configure the config file under ~/.ssh/, here is an example:

Host sing11
  User            wanghao
  HostName        sing11
  IdentityFile    ~/.ssh/id_rsa

Host sing11
  User            git
  Hostname        sing11
  IdentityFile    ~/.ssh/gitlab_id_rsa

  #Hostname        192.168.1.11
  #IdentityFile    ~/.ssh/id_rsa