diff --git a/src/frontend.c b/src/frontend.c
index 52b5bfb..1205650 100644
--- a/src/frontend.c
+++ b/src/frontend.c
@@ -173,7 +173,7 @@ void parse_args(int argc, char *argv[], arguments_t *arguments) {
       server = arg_strn("s", "server", "<SERVER>", 0, 1,
                         "address of server to connect to"),
       port = arg_strn("p", "port", "<PORT>", 0, 1,
-                      "port of server to connect to (default 5000)"),
+                      "port of server to connect to (default 45011)"),
       file = arg_filen(NULL, NULL, "[FILE]", 0, 1,
                        "filepath for read/write ('-' to read from stdin)"),
       end = arg_end(20),
diff --git a/src/network.c b/src/network.c
index 154da44..93601f6 100644
--- a/src/network.c
+++ b/src/network.c
@@ -19,7 +19,7 @@
 fd_set testfds, clientfds;
 char *msg_buf;
 size_t msg_size;
-int port = 5000;
+int port = 45011;
 int fd;
 int sockfd;
 FILE *sockstream;
@@ -29,6 +29,8 @@ struct hostent *hostinfo;
 struct sockaddr_in address;
 struct addrinfo hints, *servinfo;
 
+const char *PROTOCOL_VERSION = "1.0";
+
 /* Connects to server and returns its canvas
  *
  */
@@ -61,6 +63,25 @@ Canvas *net_init(char *in_hostname, char *in_port) {
 
   sockstream = fdopen(sockfd, "r+");
 
+  // "negotiate" protocol version
+  char version_request_msg[16];
+  snprintf(version_request_msg, 16, "v %s\n", PROTOCOL_VERSION);
+  if (write(sockfd, version_request_msg, strlen(version_request_msg)) < 0) {
+    perror("version negotiation: write error");
+    exit(1);
+  }
+
+  if (getline(&msg_buf, &msg_size, sockstream) == -1) {
+    perror("version negotiation: read error");
+    exit(1);
+  }
+  if (!(msg_buf[0] == 'v' && msg_buf[1] == 'o' && msg_buf[2] == 'k')) {
+    eprintf("Failed to negotiate protocol version: the server says '%s'\n",
+            msg_buf);
+    exit(1);
+  }
+
+  // receive canvas from server
   getline(&msg_buf, &msg_size, sockstream);
   char *command = strtok(msg_buf, " ");
   if (!strcmp(command, "cs")) {
diff --git a/src/server.c b/src/server.c
index ce10c42..fc35869 100644
--- a/src/server.c
+++ b/src/server.c
@@ -25,6 +25,8 @@
 static _Atomic unsigned int cli_count = 0;
 static int uid = 10;
 
+const char* PROTOCOL_VERSION = "1.0";
+
 #define MAX_CLIENTS 100
 #define BUFFER_SZ 2048
 
@@ -154,6 +156,35 @@ void *handle_client(void *arg) {
   print_client_addr(cli->addr);
   printf(" referenced by %d\n", cli->uid);
 
+  // protocol negotiation
+  if ((rlen = read(cli->connfd, buff_in, sizeof(buff_in) - 1)) < 0) {
+    printf("version negotation: error reading from socket\n");
+    goto CLIENT_CLOSE;
+  }
+  buff_in[rlen] = '\0';
+  strip_newline(buff_in);
+  char* cmd = strtok(buff_in, " ");
+  if (cmd == NULL || cmd[0] != 'v') {
+    printf("version negotiation: client command not 'v'\n");
+
+    goto CLIENT_CLOSE;
+  }
+  char* client_version = strtok(NULL, " ");
+  if (client_version == NULL) {
+    printf("version negotiation: unable to parse client version\n");
+    send_message_self("can't parse version\n", cli->connfd);
+    goto CLIENT_CLOSE;
+  }
+  if (strcmp(client_version, PROTOCOL_VERSION) != 0) {
+    printf("version negotiation: unknown client protocol version: '%s'\n", client_version);
+    send_message_self("unknown protocol - supported protocol versions: ", cli->connfd);
+    send_message_self(PROTOCOL_VERSION, cli->connfd);
+    send_message_self("\n", cli->connfd);
+    goto CLIENT_CLOSE;
+  }
+  send_message_self("vok\n", cli->connfd);
+  
+  // send canvas
   sprintf(buff_out, "cs %d %d\n", canvas->num_rows, canvas->num_cols);
   send_message_self(buff_out, cli->connfd);
   printf("sent canvas size\n");
@@ -199,6 +230,7 @@ void *handle_client(void *arg) {
       send_message_self(canvas_buf, cli->connfd);
     }
   }
+  CLIENT_CLOSE:
 
   /* Close connection */
   close(cli->connfd);
@@ -261,7 +293,7 @@ int main(int argc, char *argv[]) {
   pthread_t tid;
 
   /* Socket settings */
-  int port = 5000;
+  int port = 45011;
   listenfd = socket(AF_INET, SOCK_STREAM, 0);
   serv_addr.sin_family = AF_INET;
   serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);